Ask yourself the reasons Why you want to have a baby... are you mising something in your life? Do you want companionship? Are you trying to hold on to your boyfriend? Sometimes these questions are behind the reasoning to have a baby so young.
However remember that even though it seems very romantic to have a baby... it is VERY hard work! it is no picnic...it is very tiring and exhausting...even for the most prepared mom!
Also remember that at 15 your body is still growing and changing... pregnancy can do different things to your body emotionally and physically that you may not be ready for... if pregnancy is something that you MUST do at such an early age ... make sure you have support, it will be crucial!