Find a good teacher. Practice and continue learning on an ongoing basis, even after you stop take lessons. Be yourself - don't imitate. Find the music that is close to your heart and fits your voice/style/instrument/ability. Keep growing. Never stop, even if you don't perform. When the opportunity to perform arises, you will be ready, if you have continued to add to your repertoire and continued to sing/play for yourself. Get together with other people to make music for fun. The more you do that, the more confident you will be when you perform.
Alice