Just in case: you don't have to buy D&D! A huge chunk of it is subject to OGL (Open Gaming Licence) and d20 System Reference Document (SRD) is a proof of just that. You might need a few sets of dice anyway, though. The world of RPGs does not end on D&D anyway and it's a cool thing to explore with friends. System like FATE is also based on 6-sided dice, free, and a great game with minimal mechanics. :D
You'll get better with practice, as with any other skill. The only tip I can give is to know your strengths and weaknesses. I can't fast-talk or lie to save my life, so you will not see complex diplomacy of courtly intrigue in my sessions. But I'm analytical and excel in making weird traps or tactical scenarios. Suffices to say that I'm not a DM who will let you underestimate even such meek monsters like Kobolds ;P. Almost had a TPK (Total Party Kill) with them on a bunch of seventh-level adventurers who didn't feel like that's going to be a challenge. rd95 will probably get a chuckle from that one. ;)
Use a few sessions or scenarios as a practice round to get to know yourself and your players' aptitudes better. Once you are comfortable with everything, it is going to fall into place.