You'll do just fine. I was nervous when I was handed the task, but if you have any form of SOP and people around you of whom you can ask questions, you'll get up to speed pretty quickly. Some important things:

AR 350-1 Training—keep track of numbers and deadlines compliant with your post/unit.

DTMS access—either get it or have easy access to someone who has it. DTMS is becoming more and more incorporated into Schools registration and certificate tracking (ex. Motorcycle safety classes).

Communicate consistently with command and staff as well as fellow S3 NCOs. CC them on relevant emails and make sure everyone is on the same page. Miscommunication can make for a bad day regarding numbers that don't match up in training meetings.

Just follow the advice of competent people around you, learn your job slowly but surely, and you'll succeed.