i'm not too sure what you mean, gain strength from dragons? you need to work with dragons, and they probably won't favour you. if you don't know what you're doing, and just want one as a pet, or to gain it's power, the dragon will know, if you;re lucky it will simply ignore you.
first look into the basics of magic, then after a few years, you can ask the dragons to accept you and work by their side. once you begin to gain their trust, and complete a few rituals, then you can begin to gain their power. you could also call upon them to aid you in your spells, it won't be much at first because they don't know you well, but you could try that. just don't summon the dragon. say a chant that calls for dragons to help instead of a god/goddess. but i suggest you study first.