Depends on the situation, blinds, your position, chip stacks..
I almost always raise high pairs (TT and up) if there are no raises in front, and reraise very high pairs.
Medium pairs can be played for set value if you can see a cheap flop, so you can often limp in with it. The chance of hitting trips on the flop are about 10%, giving you great implied
odds. But there are also situations were you can call a raise or make a raise yourself with it, maybe you can give some more specific situation were you want to know more about?