|There's no bridge over these troubled Theatrical Waters|
#1 Not keeping your stage manager/design team in the loop
It's so easy as a starting director to think that you are God in this show and start making all kinds of intricate blocking and imagine all of these light cues and music coming at key musical moments. You may think that as the director you have the right to add these things at a moment's notice or look at offending parts of the design and just say "get rid of that".
But remember your Stage Manager has to call the show and your design team reserves the right to make sure their artistic vision is protected. If you aren't talking about these things with your team and coming up with answers together then you take the risk of walking out on a ledge with no one to support you. If your stage manager doesn't know how to call the cue and your lighting designer won't light it you don't get it in your show.
Quick Fix: Encourage your design team to sit in on rehearsals with you as much as they can or have production meetings in the rehearsal/performance space. Sometimes designs are set well before the show starts rehearsals but if you bring in the people to the places where you have these ideas and you can talk about them in terms of the actual space they can get behind you. And spend the first five minutes and the last five minutes of rehearsal with your stage manager.
#2 Giving orders that won't be obeyed (or will only be obeyed grudgingly)
In your first directing gig you probably won't be having a professional setup where all of your actors are paid and you have a bit more leeway to throw your weight around and have people obey your every whim. You'll probably have people who are donating their time for a community theater or church function or they're in some kind of extracurricular school project. It's very tempting to start getting very authoritative and demanding that everyone get warmed up before rehearsal and that actors start cutting their hair or quit bringing in food to the rehearsal hall. They may obey you and it may work for a while but you may be shocked to see that they'll start doing it anyway (or not do it however your case may be). You can just keep shouting orders but after the third time everyone will know not to listen to you.
Quick Fix: Look around and try to find the reason for the disobedience. You may find that the person bringing in food all the time at rehearsal is coming straight from work and this is the only time that they'll be able to eat dinner before they get home at ten in the evening. If they're hungry they're not putting all of their mind into the play. Instead yelling at everyone to be warmed up before rehearsal find a fun warm up that all of you can do together.
Look for part two next week.