For most good video editing software, you're going to need to buy or at least torrent stuff. Adobe CS [Premiere Pro and After Effects] are generally comsidered the best, or close to it.
For a beginner, iMovie is servicable but don't get too attached to it unless you only want to make simple edits. Not sure it runs on anything but an Apple OS, though. I only mention it since it's better than Windows Movie Maker. WMM is kinda a joke to filmmakers. It's very unstable, has limited functionality, and doesn't render very well. If you buy anything, don't buy WMM, it's okay as a starting point for free, but most other options have higher value, albeit higher cost as well.
All that said, if you're just pasting clips together and syncing clips, it shouldn't matter too much what you use, honestly. It depends on how much you're putting into the video itself.