hey, my friends just asked me to DM a game of something! but we haven't decided what system or setting or anything. What are you recommendations for something 1. playable over skype/online 2. lightweight and 3. good for a beginner DM?
Those three things are almost always what I want in a game, so you're in luck! I would really recommend one of two games--coincidentally, probably my two favorite RPGs.
Dungeon World - Dungeon World fits all three criteria to a tee. It's very lightweight by design. Everything you need to know as a player is listed in your playbook, almost everything is based on the same mechanic (2d6+Stat, 10+ is a success, 7-9 is a partial success, and something happens on a 6-, not necessarily a failure). It doesn't use minis, grids, or anything like that, making it a great online game. And it is PACKED with some of the best DMing advice you'll find--I learned a ton from it, as someone who had DMed for years before reading it. Seriously, I'd recommend reading it whether or not you end up playing (which is easy, since the whole thing was released under Creative Commons) just for its DMing advice.
Fate Core (or Fate Accelerated) - My favorite RPG of all time, full stop. Fate Accelerated is a little easier to teach than Fate Core, since the rulebook is 40 pages, but they're both really great reads (I like Evil Hat's description of the two as "FAE gives you the rules in as little space/time as possible. Fate Core takes more time to read, but it gives you the why BEHIND the rules). A little tougher to do over Skype, because of how Aspects work (it helps to see them laid out in front of you--which you could easily do via Google Hangouts or Roll20), but it's still really lightweight, and intuitive, and easy to pick up as a DM (like Dungeon World, it's all built on the same set of mechanics--an idea they call the Fate Fractal, or Bronze Rule.) Fate is also available as a (really-nicely laid out) SRD, and is also a Pay What You Want PDF. Added bonus, Fate has, as my brother once described it, "the best character generation [he] ever took part in."
Those are my two initial recommendations. I'd say look those over, see if they feel "right" to you. If not, let me know what turns up your nose at them, and I can probably throw out some other recommendations.
And if so, I wish you happy gaming!
(wow that turned into a wall of text didn't it)