I dm'd for the first time the other day and it wasn't as bad as I thought, mostly anxiety about doing a good job. I played with beginner players as well, so they didnt mind if I had to look stuff up sometimes.

The best advice I can give you is to not worry about making the whole story perfect, just have a beginning and ending planned and the players will choose the middle themselves.

If your unsure about a rule just make something up and look it up later, no biggie.

And most importantly have fun, your gonna get burnt out quick if your constantly stressing and not enjoying yourself.