I would recommend you use IMovie first. It's much simpler and easier to use and can produce very good projects. I used it myself for quite a while. It does everything you need pretty well and it's just so straight forward to get along with.
Once you have fully exhausted imovie, know all its features etc, if you feel like its limiting you, or you want more special effects etc, then that's the time to move to final cut. Final Cut Pro X is a step up from imovie. It's a natural progression. Because you will know imovie well, you'll find you can use FCPX pretty easily and can concentrate on learning the more advanced features etc. Its a much easier step to make.
FCPX is ultimately where you want to be, but considering imovie is free start with that first and learn it well before you move on to Final Cut Pro. On the other hand, you may even find imovie does everything you need it to do for your content in the first instance so you don't even need to spend the extra £200 for final cut! There are several pretty large youtubes out there that only use imovie alone!