Ok, good. Use wood glue and clamp, make sure they stay perfectly square. Measure diagonally from corner to corner in each direction, should be the same if square. On larger frames they can take a brad nail along with glue. Need to predrill a small hole first so not to split the wood. Good luck, it will work out!
First you need to decide if they are worth it. There are tons of frames at goodwill and thrift stores for next to nothing.
How are they broken? Corners? In middle? Have you got pictures?
In most cases, glue and clamp, but you don't say material of frame!!!