It really depends on where you expect to be by the time you are 21. Most BLers I know who have been here since they were your age are at best burnt out by their early twenties and tired of drug use or at worse have developed a proper addiction and wasted the best years of their lives. Some of them have even posted in this thread. So far though they haven't told you about the problems they had in school, the relationships ruined with their family and the wasted opportunities that they are now trying to catch with their friends who didn't get so heavily into drugs. Check out the Dark side and ask the really lost and miserable how young they were when they were consumed by drug use.
I'm not saying you can't experiment, but at your age the drugs you should be experimenting with don't really require that much knowledge regarding HR. If you are smoking weed or drinking alcohol don't drive, fight or eat xmas dinner with your parents. That's about all the HR a 15 yr old needs to be honest. Your brain is too young for LSD, MDMA, coke or shrooms, but if you want to experiment keep it to a minimum and wait until you are older and have cash to burn and enough freedom that your high isn't ruined by fear that your parents will bust you. Parents are more of a buzz kill than any comedown you will have. As for the rest of the RC's out there, I would personally avoid them no matter how cheap and readily available they are.
To sum up I would honestly say you are too young to invest much time here. At your age you should be experiencing everything in life. BL can become an addiction, and one that will stop you developing other healthier interests that will be much more valuable as an adult. Do you know how to sail or rock climb? Can you play an instrument in a band? Are you a confident public speaker and do you know more than one language? By all means lurk and contribute around here but for god's sake don't spend the majority of your youth here.