ANSWER - If I'm honest you have the kind of attitude that will make a good witch. I get a little tired of people who believe witchcraft is all about making the world a lighter brighter place. Yes there is a place for positive altruism but at the heart of most successful witches is exactly what you spell out in your question - the desire to bend the world around them to suit their own purpose.
I wouldn't say witchcraft necessarily goes against the teachings of God - although that does depend very much on which God you believe in. Making the world work for you doesn't necessarily mean you are going to go out of your way to cause harm to others. I think it important that if you do decide to follow the path as a witch you work out early on how this is going to harmonise with your personal faith. Witchcraft can work with or without a faith but when it won't work for you is if you choose to split yourself in two because you can't see how your religion and your craft can achieve cohesion. Establish the limits in your own mind from the beginning in terms of what you are comfortable doing and what you are happy will not work against the God you believe in. You'll be setting yourself up for a lot of stress if you don't work within the confines of your own beliefs.
I've said that most witches are fundamentally self interested and that (in my opinion anyway) is generally true. However most people on the path of a witch are looking for general ways to make the world work for them. The dedication involved in getting results out of practising as a witch will require you to have more than one single goal. So what I am saying is if you want to become a witch because you believe this is the path for you you will stand a far greater chance of success than if you want to be a witch to manipulate one specific situation to your advantage. Or put very simply - do this for you, not for the girl.
Views differ on how to start walking the path of a witch. Some witches will advise study, others to join forums to get to know other witches and still others will suggest you get your hands dirty with some practical experience from day one. I'd recommend a blend of all three - join a couple of forums and get some different perspectives on witchcraft and what it means to different people (we live in the age of technology we may as well use it...) download some pdf files on beginner books on the craft (link here) and when you've read about some of the basic techniques involved in casting, have a go. See if it appeals to you.