Mustangs are not for the novice.
If you are just getting started, it's best to wait on getting a horse. Look up local stables in your area and sign up to take some lessons. If all goes well and you like it, your instructor will be able to help you find a suitable horse to lease, which will give you an idea what ownership is like.
This is really a very big question, and I apologize if my assumption that you are a novice is incorrect - it is just that you did not provide much information.
Let me know if you have any more questions on the subject as I would be most happy to answer.