About infantry battles I cannot find a scale for scenario difficulty, since all infantry centric battles are challenging. The best I can recommend here is read the manual on how you should move your men. About tank battles, I'd say play the tutorial at first if you havent, then go to battles that you have a clear superiority in quality, quantity or both (Desert Storm battles are the best here, try to do the additional objectives, like not losing a single unit, or capture all the objectives in 10 turns etc). When you feel like it, go to hypothetical cold war scenarios from the NATO side. You still have superiority tank-to-tank, but you wont have the initiative, you'll be outnumbered and outgunned in terms of artillery, plus you'll be attacked from the air. A lot. After that, go to cold war scenarios from the WarPac side. You'll need to be good at combined arms warfare in order to accomplish these scenarios.
Oh and to finish my post, when you're ready play scenario #257: East walkers. I tell you, plinking M1 and M1A1 Abrams with T55s is the most satisfying thing you'll ever feel in this game...
