Using a paintbrush is a basic part of our hobby, but just like other skills it needs to be learned - that means getting in some practice.
Many people wash their unpainted model first to get the mould release lubricant off, and to remove fingerprints and sanding dust that always seems to accumulate no matter how careful you are. I use a small jam jar filled with warm water with a few drops of washing up liquid. A cotton wool bud makes a good tool to wash down the model without getting it too wet, and be careful not to snag aerials or brass bits. Once the kit is dry you can blow off any dust (I use an old clean and dried washing up bottle to puff air at my model) Now you are ready for that all important first coat of paint.
There are Tamiya acrylics, and Humbrol Acrylics and Enamels and Vallejho Acrylics, and Xtracolour enamels, and Humbrol enamels... The list just goes on, and that's just a few of the ones available.
What type of paint you use is up to you. I have good reasons to use enamel paint - I've got used to it, its easily available and the colours are repeatable. But do take a look at Acrylic paints too before you commit yourself to a particular type of paint only. As a pointer for now, Acrylic paint is rather thinner in my experience, doesn't always cover with one coat, and can be streaky. BUT it does have the big advantage that it is very unlikely to react with other paint, it's a 'cool' paint and is very well behaved. In contrast enamels are rather 'hotter' and can cause difficulties if over coated with the wrong top coats or varnishes. Lacquer paints on the other hand are so hot that they can actually destroy plastic, so be careful..!
One trick you might like to try first (especially if you have a multimedia kit with plastic resin and brass parts) is grey or white acrylic primer in a spray can. This can be bought from Car Accessory shops (Halfords in the UK is a good place to try) This gives a key for your paint to stick to and overcomes the streaky finish you can get sometimes on bare plastic. The chief disadvantage with this is that you can't really spray indoors - leaving you to venture out into the garden at weekends and hoping for good weather with little wind. There is a great deal of overspray, and you shouldn't inhale the fumes. The propellant can be an irritant too if you are sensitive - although Acrylics are the most friendly spraycans around.
For detail painting I use enamel paints un-thinned. I tend to use either Humbrol or Revell paints in the little tins, because they are more easily available. There are others and good ones too - I've used a number of them, but they can be difficult to get hold of on a Sunday afternoon! If I'm doing a complete AFV or airplane model I tend to brush the first coat un-thinned, and then thin the next coat - I find the thinned second coat seems to go on smoother. If you want to try this get some of those translucent 35mm film tubs, beg them from a photographic shop - you should be able to get a dozen or so. Use them to mix your thinner to paint ratio based on the tin instructions, or the datasheet provided by the manufacturer. If there is no data sheet available or if you have any doubts you'll need to experiment with a drop or two first, then increase until to get a result you like. Practice on old kits, or bits of plasticard rather than on your model at first.
If there seems to be a bewildering variety of paints - The choice of thinners seem to be even worse... A lot of people will swear by home brewed concoctions - citing things like red or blue windex (window washer fluid), Isopropanol, distilled water and more exotic mediums like Liquin and turpentine that are intended for oil painting.
My advice is simple. Anything you add to your paint will change the way it dries and its final colour. It may also change whether it dries shiny or matt. So, in general you should use the same brand of thinner as the paint you are going to paint with. The only problem with this is the tiny bottles of thinner, and the high price charged... But - you should also experiment to find what suits you best. I use isopropyl alchohol (available from some chemists) with a few drops of the branded thinner. Too much will make the paint too runny - the exact amount is a matter of personal preference
I like to get my brushes from two sources. One is my local model shop. Here I've got the choice of Revell, Humbrol and Tamiya hobby brushes. I usually get large brushes, either number 5 or 6. These will hold a fair amount of paint and will allow smooth and steady spreading - minimising streaking and air bubbles.
For more detailed working I buy a few good Sable paint brushes. Brands like Daler and Pro-Arte are available at art shops, or at Hobbycraft if you have one near you. They do cost a fair amount, but if well looked after they will last a while. I have size 3, 2, 1, 0 and 00 - and a couple of 0/5. (Yes that's 00000...)
Well it's for a vital part of making the model look real. and I'll tell you after this bit. First paint your model with a number 6 or even bigger in the basecoat colour, and leave it a day or so to dry. You may want to give two thin coats to ensure proper coverage. One thick coat will never look as good - believe me I've done it and regretted it! You can use either Acrylic or Enamel for the basecoat, either way make sure it is properly dry.
Next, get a bottle of the modeller's friend Johnson's Klear. With the largest brush you have - at least a 6 and preferably an 8, and preferably one kept for doing acrylic varnishing only. Brush a thin coat of this wonderful substance over your model. It's an Acrylic Varnish and a coat or two will seal your basepaint away from the wash you are about to add. As it's Acrylic it will sit happily on enamel (once properly dried) and won't be affected by any further coating with acrylic type paints - or drybrushing with enamels.
After your Klear coats have dried - I give it two full days and put the wet model under a plastic fish tank I keep for the purpose. This keeps off the dreaded dust and - in my house - cat hair... Once I'm certain it's dried, I make up a thin solution of water and acrylic paint. And I do mean thin - at least 10 drops of water to one drop of paint. I then dip the brush into the paint and touch the wet brush to a panel line or hatch cover. As the model is now smooth and slippery the water will be drawn into the narrow gaps by osmosis - taking the paint with it. If you choose a darker shade than the base coat - say black on Panzer Grey or Panzer Grey on Desert Sand the details will pop out. As the water dries off it will leave the paint in places you just can't get a paint brush…
You can't do this with enamel paints - which is why I suggested trying Acrylics as well as enamels... After your wash is finished you can coat again with Klear and then apply your decals. Another coat of Klear will make the decals part of the base coat. Then you can dry brush with enamels in the normal way and create your final finish - safe in the knowledge that your original basecoat is safely protected.
This wash technique also can be made to work on your base coat - before you put your layers of Klear on. In this case as the paint is rough, your drips of watery paint will dry where they are put. This means that if you have a dark panzer grey or black once you have dripped it on and let it dry you have a pretty convincing oil stain that becomes a part of your original base coat. Put Klear over it and then dry brush your enamels over and the oilstain will fade, but still add to the used look of your machine.
Similarly, a rusted piece of bare metal can be made by over painting with a dark red basecoat, say Humbrol enamel matt 113 brick red, letting it dry, then dripping ordinary water on, and then dripping some thinned matt orange acrylic on top and letting it dry back. That way you even get drying rings as the water evaporates