As Russia you need time to build up your defences and to weaken the enemy. You must pull as much infantry from the east as possible but leave a small rearguard there. I would suggest you buy some tanks or fighters quickly so you have more attacking power. You can trade infantry for losses while keeping those tanks and fighters alive as much as possible.
It is essential that Russia wears down the Germans as much as possible by giving up territories and then counter attacking. In the meantime the Allies must start knocking on the door of the Germans, so that they are forced to divert troops for defence. This will hopefully slowly give you the chance to build up powerful armies that can kick back the Germans when they really start threatening Moscow.
Whatever happens, Moscow must not be allowed to fall. You must keep always at least one infantry on the surrounding territories so the Germans cannot Blitz to Moscow with a huge tank stack.
Playing with Russia is very challenging at first, but once the worst danger is over you can really build a huge Red Army and conquer pretty much all of Eurasia. I very much enjoy playing the Russians because it really is mostly brutal land warfare with huge stacks of Inf. Tanks, Arty and Fighters. Very basic and simple, and very bloody.
Cheers!!