get started by surrounding yourself with players better than you.
in a newbie mix you will have 1 coach that will teach you the game with other people in a similar situation as you. games are meant to be fair and are organized based on experience.
you can also play tf2 lobbies. please play a lobby and be sure you can finish it. there you can practice what the coaches have to told you.
the other thing to this is DM. DM servers are the best play to get better. DM MOD can recreate the 5v5 fragging classes scenario in the same map with spawns close to the MID of the map or is centralized around another point depending on the map. for example in gravel pit the point is centralized around B point. The 1v1 mge form will not help as much as getting raped playing all the good players in the dm server at around 7-11pm on weekdays. the harder you get raped the more REAL the more POWERFUL the experience you are having toward getting better. before you know it you aren't getting raped.
the next step after you up your DM a bit and get used to the 6v6 format( from tf2lobby+newbiemix) is #tf2mix. This is a irc channel on gamesurge where you can play 6v6 format pugs.
The next step after you up your DM a bit and get used to the 9v9 format is #tf2.pug.nahl.
this is also a irc channel on gameserge where you can play 9v9 pugs
the next step after you feel confident in tf2 mix is #tf2.pug.na
You can access all of these channels on a website called atf2.org. its very simple!
if you have any questions you can add me on steam!
=D