It is easier to develop a good reputation if you have a specialty and become known for your skill in that area. The old saying, “Jack of all trades, master of none”, is true. Mastery in a specific area will help your clients recognize and appreciate that skill.
Don't promise something that you can't deliver. Be honest. If you don't know how to do something, tell the client and don't take that job. If you think you can do something but have never done it before, inform the client and let them decide whether to contract with you.