Don't be so hard on yourself! Everyone has to start somewhere...
Just have a set of answers for everything, it's not hard!
Ok first off - no customisations. If you aunderconfident DO NOT allow them to be anything which is not in the book.
Do not allow them to be aliens, machines or anything which says "It is not recomended you allow player characters to play this race"
It is easy to stick to the standard "standard set of officers on a ship or station" but then look at DS9 - a remote setting in which the Starfleet was in technical dominance, and not far from home for backup, but there were also a number of non Starfleet main characters who involved themselves and powerful aliens.
Ok the first thing you should start off on is - find out what they want to play. If they don't want to play a squeaky clean TNG adventure, suggest either TOS or DS9 Style - the past has penalties because they don't have UBER technology. A DS9 style setting is a ballance problem.. how would you have Quark in EVERY episode!
Well I actually braved out that scenario and it worked out as one of my best and most successful campaigns. The Crew are stationed on a requisitioned vessel (form some pirates who attacked them!) and it consists of a mix of Starfleet and non starfleet... Ok how on earth did this wierdness happen?
Ok, well when I began not eveyrone wanted to play Starfleet so I thought how can I? The setting is post dominion War, and the players are located on a very remote Starbase on the Edge of the Federation. The Choice was simple - if you were a Non Starfleet officer you worked FOR starfleet - you could be a smuggler a thief a Warrior or a Mystic - anything you could envisage, but you worked at the Starfleet outpost.. why ? Starfleet has a serious staffing problem post dominion war, so they often take on 'crew' to get things done. You could be a scientist and work in the science department, if you were a warrior - you worked in security.. The upside for such players is they have FAR more personal freedom, and can technically have 'night jobs' You get good pay and accomodation, and everyhting is FREE! The Down side - you can only access what the Fleet officers say (Computer information, controll panels and rooms) - You can only replicate what you are allowed (So no phaser bombs or home made explosives) and you do what the Starfleet officers say: SO yes you are technically not bound by the Prime directive - but THEY are, and if neccessary, just like everyone else you can get vapourised if you treat it flagrantly !
So long as the boundaries are clear, they can probably play the character they like from the shows, and it makes them less unrully because they aren't playing something they don't want too.