First of all, the 5th edition rulebook is rather verbose ('tis a book Gary Gygax would be proud of ....), and the details of the rules are often buried someone in paragraph or two of text. So allow plenty of time for reading before you start anything. Consider flagging things with post-its, or even defacing your book to highlight where rolls are defined. The rules overall work quite well, IMO, but finding them all, interpreting them in places, that can be a bit of a task.
When it comes to character creation, I strongly advise talking with the players and agreeing on character objectives. The system plays a bit like many point buy systems, where you can be super good in one area if you want, at the cost of versatility and balance. I'd almost say 'make some super specialized characters for one session, just to get it out of your systems, then go build the ones you'll play for a long time.' Anyway, up to your group how you want to play, but I do suggest guiding them or coming to a group agreement. Also to note: the sample characters are rather low specialization, because they were built with a skill restraint that is not in the rules as printed (something that was dropped during development). there is a thread on here where there are more 'competitive' variations of those builds. Oh, and making a competitive technomancer in 5th is hard, so guide players away from that unless they really want to do it, in which case the player and/or you should do some reading up on these forums on the topic, to avoid intense frustration.
Shadowrun is a lethal system if characters want to have stand up fights. It is one of those systems where as you take damage, you take penalties to nearly everything you do, so once you start losing things can get bad quickly. Think of detective or spy movies, there may be some violence, but almost never set piece battles. Combat may fill a lot of the rules, but the goal of the characters should usually be to avoid combat as much as possible (and therefore they need skills to do that).
The game can play anywhere from almost action movie to super-serious-spy-vs-spy, first wrong move you are toast, style. You need to decide on that tone, and make sure the players are on board.