Everybody's giving completely different advice, which is very confusing.
Second hand kit is a good idea, for something which is likely to be bashed to bits. If the OP really wants to get the hang of something properly, simple, new, unused kit is better. Speakers are the only thing which sound better with time, and in terms of sound per pound, the B&Ws are going to give better sound than anything even second hand (believe me, they're incredible for the price).
There's no nice way to phrase this but you have no idea what you're talking about. Most people who buy hi-quality hi-fi gear that's suitable for monitoring will do their best to preserve it, furthermore if the build quality is there a lot of it will outlast cheaper modern offerings. Secondly burn in is a hotly contested area, but it's fact that transistors and other component's properties change as they are used. Cable and speaker burn in is much less provable. On the subject of the B&Ws, I seriousely doubt they are going to beat anything from the kef reference series, a lot of which was developed for the studio and sold at prices ranging into the thousands (£s) and now available for a quarter of the price. However I can hardly claim that they would be any better than something I've never heard.
Anyway back to the op. Sounds like to me you should get a mic (I'd go for an sm-57, a classic mic, a total workhouse and very durable) and an interface and not worry too much about the quality of your monitors for now. If you can already play keys and you are interested in music I'd get a microkorg or something similar. IMO Learning to play guitar is a must for any 14 year old (I learned about then). Get yourself a squire strat or something cheapish to start with and watch the opposite sex line up. No really...