I live in the UK ... Do you need to be a member of a federation?
Yes you do. You need to become a member of the English Chess Federation. You can do that on their website. You either need Silver membership, if you are going to play congresses only, or gold membership if you want to play FIDE rated tournaments as well.
How to find and pick the correct event
Again the ECF maintains a calendar of events on their website. There will be events that don't appear there but the vast majority of events in England will be listed. Events will normally have different sections according to playing strength. Enter the section that best reflects your level.
It is probably also worth your while to find the nearest chess club to where you live and get in touch with them. They will have lots of good advice, particularly when it comes to local events. The ECF have a club finder page which can help you.
How to prepare for the event.
That is really far too general a question to be answered here. I would edit your question, if I were you, otherwise it is likely to get closed for being too broad :-)