Its a shame we don't have newbie mixes anymore. It gave brand new players some structure and you played with people who were in the same boat as you. I used to run a lot of theses some years ago and we used to have teams formed from the mixes which was really good for the scene.
If you want my advice i would say this:
EDIT: If i was to start playing TF2 fresh in the scene i would try to actively find a team in open as quickly as possible. When you get some league experience under your belt it gives you an idea of what is needed from you to be a good player. Watch demos or stv's of the best players. Consume as much 6v6 TF2 as you can. Unlike when i started there is a huge amount of resources for you to help you improve. Its all been linked on this thread.
Tf2 center is basically pure trash tf2 and I would avoid it at all costs. It has a really awful player base of some of the most toxic players I've ever met in TF2.(and that's impressive in this community) and new players will just get slammed by stacks or shitalked by their own team.
Face it is slightly better but still not ideal for new players. Every time I play face it I am put on a team with Russians who cant communicate in English or don't say anything at all. People give up after a round loss throw or just troll the entire game. Sure you might get a good game with communication every now and again but it is rare. It was potential to be amazing. But ironically because of the low player count in matchmaking the worse off it is. Which is why if you are going to play some semi organised TF2 you should jump in the queue. Unlike TF2 center I wouldn't avoid it like the plague.
Honestly I would play Mixchamp (if its still active) if you aren't picked on pugchamp or play Pugchamp at off peak times. They are at least organised and for the most part not toxic bad experiences.
EDIT: You should play MGE and jump maps if you plan on playing in 6v6 in my opinion. It gives you control of the game and you get used to certain mechanics which may not be apparent in pubs. All classes benefit when you grind those out.
I've been thinking about starting newbie mixes again. So if anything happens on that i'll make a post about it.