I play the Double Bass and I've always wanted to compose music for an orchestra. I know that I need to learn more theory, but I don't know where to learn it. I was thinking about learning it in a book, is that a good starting point? Also is it beneficial to learn the piano to compose and understand chords?
Learning the piano would be a good place to start. It will get you more familiar with the treble clef and how multiple voices interact as well as harmony. (This should also give you great benefits on the bass, too.)
A second important skill is musical handwriting. Get fast at making the dots and stems on staff paper. Write down all your little melodic ideas in a notebook which you can refer to when you're writing something and stuck for ideas.
Imitate your heros. Take a piece that you like and try to copy all the tricks and ideas that it uses but built upon a different melody (your melody).