You don't need to play an instrument in order to write for it. After all, the vast majority of people don't play every orchestral instrument (at least not well; music ed majors typically have to learn a bit of everything but they're not expected to achieve anything close to mastery). It's just important that you know how every instrument works so that you can write for it effectively. It's surprisingly easy to write something insanely difficult without even realizing it.
Pick up a book on orchestration. There are lots of good ones, and one of the standards is the Rimsky-Korsakov Principles of Orchestration, which is public domain. A few bits are outdated (don't write for alto trumpet) but you won't do anything too bad if you stick to the guidelines there.
Most importantly, run your parts by good musicians (or just post here) to make sure nothing is terrible.