Having problems with our sunroom...?

The previous owners didn't build it correctly, so it's not the best sunroom, however it works. The problem is that it is leaking. The roof on it is flat, It looks to be sealed well with (on the roof) but when it rains water must be getting in because between the panels it is damp & a green mold mildew is forming! Help! How do I fix this without calling a prof.! (don't have a lot $ and I am a big DIY!