Lot's of good advice in here. All I'll add is if you're serious then build levels, look for good mods, etc. to join, find ways to get your work noticed - look for level building competitions, etc. and don't just play games but dissect them. When you play a level consider what works, what doesn't and why.
You might also want to try and make connections - you're young but it wouldn't hurt and might help you really get a feel for whether this is for you or not. I've exchanged emails with best selling authors, artists as well as a number of game designers - you'd be surprised how often a well written and polite email gets answered. Use the internet to confirm level editors whose work you like and drop then an email if you can and try and get some feedback from the industry itself.
As SamuelRSmith says think through all the options.
Try to be reasonable... its easier than you think...
