Since you play the double bass, you understand music notation, so that is a good start. As you say, you need to learn music theory as, in its various forms, it is the basic materials fo composition. Some theory you may already know but here are my suggestions; 1. Scale and interval construction, 2. Chord construction, 3. Basic four part writing, 4. Advanced harmony. 5 Counterpoint, 6. Forms and Analysis. This a thumbnail sketch of music majors learn in a college level music program. You will be able to start doing some composing with any of these subjects but will find that they all lead comfortably to the next steps as are laid out above..
I can help you with this if you and/or you can get books on any of these subjects. I do think it's helpful to have someone to guide you and answer questions that will arise.