play newbie mixes ofc
watching streams or vods of ppl playing 6s (at any skill level really) is a good way to get a sense of what the game is usually like, and continue to get ideas for new plays/better positioning. lots of the top tf2 twitch streams from like 930-1130est will be people playing 6s at various skill levels. if you have particular players u like there's youtube channels that upload old vods from lots of top players, easy to find if you just search the players name
if you have a good idea of what class you want to play theres also plenty of good videos going over the basics (rollouts, positioning, plays, jobs, callouts etc) usually per map. some will be better/more updated than others but at a low level most advice is useful (just dont take everything u hear as permanent gospel obv)
best way to get experience just playing the game without committing to a team is to ring (play when a team has a player absent) or join a team as a substitute in Newcomer (or Amateur, depending on your skill/confidence). both can be done on the RGL discord (check/post in the "Recruitment and Scrims" channels)
if you feel stuck, lost, or cant figure out what youre doing wrong, having a mentor to ask questions/watch your game povs with is very helpful (can also be found in the RGL discord)
dont be afraid to ask dumb/smart/newbie questions to any more experienced players you know