I just air brushed a coat of olive drab on an M41 Walker Bulldog. It looks pretty good. I want to do a wash but am afraid to ruin it. I have read about how to do it but I need one thing cleared up. When doing a wash do you just brush it all over the tank, or do you just put it in the areas you want it and let it run in the crevises that way?

I understand your worries...especially with your first tank. No need here...
Did you apply the decals yet? If not, great! If so, still no biggy, just follow below.
If you can, apply a coat of Future Floor Wax (or if you prefer a coat of another's gloss) to the model with your AB. Let this dry a day. Apply the decals, let them set for a couple of hours, and apply another thin coat of the gloss. This second coat will help protect the decals from the wash. Let this coat dry another day.
You can wash the entire vehicle if you want but, this could darken the whole vehicle while still "dark-lighting" the recesses and other details.
The better way, which requires a little more patience, is to apply the wash the specific areas on the model. With your brush wet (not too much though) with the wash, just touch the tip of the brush to the areas of concern. The gloss finish will assist in the capilary action of the wash "flowing" into and along the detail areas. If too much wash is applied, you can use a soft tissue or similar to soak up the extra.
When the wash is done, apply a flat or dull coat to seal this in and move onto the drybrushing if you so choose.
I hope this helps a bit.

No I have not put the decals on yet. I read the burnt umber is a good color to use for a wash.

Burnt umber is great for a wash although the olive drab opens up other options such as a rust coloured wash. It should be thinned about 70% thinner and 30% paint give or take.
Just follow Kencelot's method and things should turn out well.
Good Luck!
Andrew

i would waite more then a day , for the gloss to dy waite a week. CAUSE I DID THAT ON MY TANK AND I RUIEND IT THE GLOSS COAT WAS NOT CURED YET AND T HE WASH RUINED THE TANK SO PLZ WAITE LIKE 4 TO 5 DAYS I WAITE 5 DAYS FOR THEGLOSS TO DRY. GUYS IN MY MODEL CLUB WAITE THAT LONG

I have another question about how to weather armor with washes. If I put the wash only on the reccesed areas and then dry brush after that then how do you get the metal on the tank to look weathered? I see all these tanks in the gallery and the armor looks weather beaten with difterent colors. Do you get it to look this way by washing the whole tank or what? I have read about using pastles and have a good set of them on hand if I need them.