i just joined. i was wondering if someone can give me an idea on per say how long it would take to get a 1st round draft pick trained up near his max value?

Depends on the player, a 1st rounder will typically be pretty much at their maximum after two to three seasons. If they bust they will usually hit that potential faster because they do not have as far to improve. Later picks tend to take longer to hit their potential.

does starting them help them develop faster than letting them play as back ups?

Not that I know of.

Only if starting them means they are on the field for more plays.
More plays = more experience = move movement of their current ratings.

I think playing time only matter for position experience and play familiarity right?

I think I've read before (maybe from JDB) that players will reach their potential faster if they are starting / getting more playing time.

It will improve their experience at the position but not their attributes, their attributes only develop through training camp and practice.

what exactly is practice? is that the game planning or something different?

Your team practices between games, basically they get the attribute growth benefit from just being on your team. But they are also practicing your game plan and practicing against your scouted plays. Setting your gameplan and scout plays are all you need to do for the weekly practice.

Practice impacts are simmed immediately before the game, so you don't have to have this in place for the midweek stage.