When partner opens 1 of a Major suit there are standard ways for you to support that suit.
But if an opponent intervenes with an Overcall the situation changes. A very good method to follow is to treat all direct raises as weak, (less than 11 points), but to let the raise indicate how many trumps you have. With 3 trumps you bid 2 of the suit, with 4 trumps jump to 3, and with 5 trumps jump to game. This has a preemptive effect but also might get you to a good contract.

If you do use this method in competitive situations, then you need to have a way to tell Opener when you have a good hand with support for his suit.

An immediate cue-bid of the Overcalled suit shows good trump support and at least 11 points - this is referred to as 'Limit Raise or better.'

Lesson available.
Also see lesson on Law of Total Tricks for more about the weak raises in first paragraph.