Find a place and time where you will not be disturbed. Decide what type of poetry or music you are interested in writing: personal, topical, historical, humorous, political, sad, culturally specific, or more general. Begin by brainstorming and writing whatever comes into your head. There will be plenty of time to form these ideas into more structured poems and songs.
You won't even need an instrument if it's poetry you are interested in. Most poetry today is written from a personal perspective, but it could also be about cultural or historical events. Try to strike a balance between writing from your heart and maintaining a certain perspective and distance from your material. Avoid cliches and overused phrasing. Give a lot of thought to who your audience is. If you write your poetry for your own enjoyment only, it will probably be quite different in terms of both structure and content than if you were preparing it for a large crowd.
If you are a musician, decide whether you will write the music or the lyrics first. Try both methods and see which works better for you. Don't be afraid to ask for tips or help from friends or other musicians. Remain humble and remember that there is always more to learn.
Poetry and music are both primarily means of communication. While you can certainly write them for yourself, they don't really come to life until they are shared with others. If you are nervous about reading or singing for other people, be sure to start out with friends who you know will be supportive of your efforts. Stay true to your goals but also recognize your limits, and allow yourself time to improve your skills and confidence.
Experiment with different forms of poetry and music. Write collaboratively with others and see what happens. You can even combine writing with performing by experimenting with ad lib and improvisational methods with an audience.
Play to strangers on the street. Read a poem on the bus. You can never know exactly what will work until you try it.