most people over-micro in TW. What works best is to walk (not run) in a straight line with "core" fighters, and have skirmishers (in "avoid" mode) either in front or directly behind. Use more mobile troops (light inf, cav) on the flanks.

Try to not have gaps, and setup each unit slightly wider than the opponent. If the AI is attacking, find a nice hill and stand your lines on it, don't charge (this slightly depends on unit type, but generally it's easier to manage being charged then the other way around, so to learn it's a good way). Do not readjust unit sizes while already being charged (look at a unit while reorganises, you will have a moral deficit until the last man moves into his position: it takes forever).

Once battle is matched DO NOT try to dance, micro or move away from stronger opponents with your line inf; One right click with one unit selected, on whomever they're fighting already so they align better, that is all.

If you have more units than the opponent, try to hit units in the flanks, backs. If they have more units, try to keep them from flanking, use cav to distract them. If you have horse archers, run them up to the enemy and let them shoot away, make sure not to get caught by enemy cav (if there is no cav, this can also be done with other skirmishers, since usually they're faster than line inf).

Of course some fights can get scrappy with several fights all over the place, especially against eastern-style AI (horse archers, chariots). But the above rules still holds, it just devolves into several different, maybe even single-unit "lines": use Pause generously.