A good first step in any experiment is just fiddle with your apparatus
In this case, this means, just building your pendulum with some length and watching it swing while making some observational notes.
- What did you build your pendulum out of (what is the weight at the end)?
- How did you mount it?
- What length of string did you use for these first observations?
- Does your pendulum swing in a flat 2-D plane?
- What happens if you let it swing too long?