There are a variety of things that can cause a shower door to leak. The edges of the frame are usually caulked to the shower walls, and that caulk may simply need to be replaced.
Check the edges to see if you can spot the leak, then re-coat the spot with clear silicone caulk. If the glass itself is loose in the metal frame, it may need a new bead of caulking.
As these doors age, the original caulking begins to crack and break down. If the doors won’t close properly, it may just be a case of loose hinges.
Sliding shower doors run on tracks with wheels, so if they don’t slide correctly, be sure they’re properly seated before spending money on a repair call.