Triarii were traditionally used as the last line, but aside from that there's a few simple concepts to keep in mind.
Try to stretch your front line to be as long as the enemies. As a Roman faction, your troops don't have much charge damage but they are very good at holding a line. Use your hastati/principe as your frontline and make sure and use their pila up before engaging. If you are on the defensive, make sure they are on fire at will so they pepper the approaching army before it closes.
Use reserve infantry or cavalry to flank at least one side of the line. When it folds, send everything you have on that side at the next unit in line and so on. You will cause a chain route.
Whenever possible without overextending yourself, try to kill the enemy commander as early as possible. The enemy troops will route a lot faster after their commander is dead or fleeing.
Make good use of velites, skirmishing in front of the enemy lines until they are close to closing with yours and then send them behind your line then to a flank to pepper the enemy with missiles from the side.
Morale is very important. Missiles, being outnumbered, being flanked and not being in proximity with their general all lower a unit's morale and makes them route easier.
Keep in mind that missiles to extra damage when coming from behind or the the right (the side without a shield).
Play some practice battles and you will get the hang of how to maneuver your troops. Others will be along shortly for a lot more tips, I'm sure. Soon you will be complaining at how easy it is to win battles.