Disable horizontal scrolling in themes?

Hello,

New here; hope I'm posting in the right spot.

I'm still building my website. Grabbed up several free themes to toy with. Some of them make the forums too wide, making horizontal scrolling necessary. I imagine the themes were designed with laptops in mind; catering to a rectangular screen rather than a square one. Is there some method by which I can remedy that and rid myself of that horizontal scrollbar?

Things you should know...

1. I have ZERO experience or knowledge with any of the coding that creates these themes.

2. I do, however, learn quickly and I'm not afraid to mess around with the code.

3. If messing with the code, I may require more lengthy/thorough instructions than most people need.

Thank you for whatever help you can offer.

~Jamie