Ok so I have applied my coats of paint and I'm ready to apply decals and weather my vehicle 1/2track.
1)Do I need to prepare the model before applying the decals or can I put them on after the paint dries?
2)What should I do prior to weathering like drybrushing?
3)What should I do prior to applying a wash?
4)What comes first? The wash or the weathering?

Hobbes

I usually coat my whole vehicle with a matt varnisch (or future but I haven`t worked with that) I do this after the whole tank is painted. Then I apply the decals, do some weathering, after that I apply a second layer of matt varnisch to seal the weathering and the decals. This also solves the problem of your decals getting silvering or shiny. then I do some washes and again some more drybrushing. this always worked for me

hope it is of some help

Thank you Faust. 1 more question though?
How do I weather tires and should I heat them to simulate the weight of the vehicle on the tires?

I usually paint my tyres with a mix of paints.
this mixture is something like black, skincolor, and some earth color. I can`t give you proportions as I mix on the eyesight. I paint my tyres and if necessary I dry brush them again with some earth color.

Sure you can heat the tyres to simulate weight but be carefull not to overdo it and be carefull for your workbench as burning rubber and plastic really stinks in the bad way of the words

I usually aply a coat of GLOSS varnish (after painting and drybrushing) before applying decals. This gives a smooth surface for the decals to adhere to. Then a coat of MATT varnish to hide the clear film surrounding the decal and give an overall matt finish. When dry I then do washes and weathering and more drybrushing if necessary. I always give a couple of days after each coat of varnish to let it dry completely before moving on to the next stage.

Couple of things -
The tires of the 1/2 track are only supporting the front end, there won't be much in the way of compression. If you are doing a diorama in the country vs. hard pack road you will notice it even less, grass and all covering it up. Plus, I've seen expensive AM replacement tires (ones with chains for snow) and I don't compression.
Isn't the tire on the Tamiya the rubber kind? As Faust mentions, I second that I wouldn't heat this kind, you'd melt the rubber and it would turn to mush. You might try the hard injected plastic kind, not the black rubber. I forget which the M3A2 comes with.

Prior to decaling I would apply a coat of gloss of some kind. This gives the decals something nice and smooth to adhere to. This keeps air from getting in underneath and causing 'silvering'. Apply the decals, then a coat of gloss again to seal them in. Then you can apply washes if you want. The gloss helps the wash 'run' into the creases. Then apply a coat of dull coat on top. I like doing that to give the dust and weathering something to 'bite'. I usually weather then apply another coat of dull coat, then apply pastel weathering last. (I know lots of options and steps).

Another note on decals- trim away as much of the extra film from the outside edges as possible. Use a metal ruler and a new exacto blade to carefully trim away right down to the color. This reduces the amount of clear film that could cause problems with silvering.

The tips prior to this are solid too, so take all together and 'feel' your way into what works for you.
Remember if you're not sure how something will look test it out on a scrap model or a scarp piece of plastic, tupperware (careful of the Mrs, or Mom) or an old fast food container.

what brand should I use for washes?

As far as flattening tires. Take a walk down town or at the mall parking lot. Look at tractor trailers. Even fully loaded, these tires show very little compression. In fact, the first thing you'll think if you see any significant compression in a tire is that it is loosing air pressure. A tire that shows significant compresson is way low on tire pressure and is not going to run well.

I paint my tires first with Floquil Grimy Black, a very dark gray, darker by a shade or two than panzer gray. I then use thinned out black paint for the wash. This is very sublte given the dark base color. Then I'll use an appropriate earth tone to drybrush the highlights. I use either enamels oor oils for the wash. You can drybrush with anything, in fact, I've found the quick drying quality of acrylics particularly well suited for this.