Learn to aim. Aiming is everything. You should almost never use auto-aim. Against TDs, try to hit the tracks. Against American heavies and some mediums, try to hit the hull (sherman is weak everywhere, of course). Against German tanks, hit the back. For the Russian tanks, good luck - their sloped armor makes damaging them very difficult.
Russian TDs are deadly but inaccurate at longer ranges and slower than german counterparts (mostly). German TDs, save for the Hetzer, tend to be death on wheels. As a light tank driver, most of the time you will be simply scouting for the artillery. Keep moving, expect to die, and track whatever you can. Honestly, I found light tanks to be less fun - I love the TDs, but TDs require a specialized sort of play. Try a bit of everything, and go with what you enjoy most.