Since you are 15 you might want to work on self. You might want to manifest better concentration, self-esteem, better grades, learning more easily, etc... These are all things that will help prepare you for your future role in life as what ever you want to be or whatever goals you set.
Manifestation covers a very broad range of possible positive outcomes for every situation in which you find yourself. When each of us begins to think and speak in positive terms that is what draws good things to us.
You might want to start with something small that has the possibility of being achieved more rapidly than other goals. Perhaps you might begin with raising your grades by manifesting that studying becomes more enjoyable or easier for you.
I am speaking hypothetically, of course, because I don't know anything about you.
The key to manifestation is that you must raise your positive energy and BELIEVE that those grades are already straight A's or a solid B average or whatever you want to raise them to.
thesecret.com provides several free tools for us to use to raise the level of our energy.
I am now recommending this one,
http://thesecret.tv/secret-to-you/ to my clients. I ask that they download this to their desktops and play several times per day. For most it immediately elevates their energy. This might be a good visualization tool for you to try.
Blessings!
C
