Make sure you are holding the horn properly. If you aren't careful you can damage nerves in your left hand and arm and that numbness will come back again and again. Have your teacher (if you have one) examine how you are holding the horn.
Absolutely. In particular, make sure that there's as little weight and strain as possible between your thumb and index finger. The medial nerve (the infamous carpal tunnel nerve) branches off there (you can easily find it by pinching the fleshy part of the web of your thumb) and you don't want any damage or inflammation there.
Also, since it's your whole arm, give some thought to your posture. Don't hunch over as you play, but get your butt forward, back straight, shoulders square, and elbows up. Just like they drilled into you in grade school. (Do teachers still do that?) You might have to build up a little more strength in your arm to hold up the horn, but it's worth it to avoid pinched nerves and arteries.
Perhaps most importantly, learn to recognize the signs of strain and fatigue and take breaks before a problem occurs. It's real easy to get caught up in what you're doing and, before you know it, you've got muscle spasms or pins and needles or chronic carpal tunnel syndrome or something.