(for batters) The first criteria I use is bench them if they aren't playing. You can tell this because it'll have their opponent/game time in the column to the right of their name. Knowing which ones actually start the game is more difficult. For starting pitchers, look for the "^" next to the opponent. For batters, click the "Research" tab, and check the depth chart. Higher probability that they will start if they are #1 for their position. This is weirder for catchers who swap in and out relatively often (see Napoli, lol). Then you can look at a bunch of other stats if you're determining who to start over the other if two players with the same position are playing that day.
1. Are they playing?
2. Stats/match up
3. What stats do you need? (hits, runs, RBIs, SBs...) and start the guys for those stats if possible, do this more near the end of the week.
Pitching: what I've found is you want to keep your good relievers active and swap out your starting pitchers based on if any of your SPs are making starts on a given day.
Then again, this is my first fantasy BBall league (or fantasy league at all) and it's pretty daunting to start with baseball, lol