What is the meta for leveling this season? Which activity?

pits to 70, then do the seasonal. no real way to boost paragon without leeching pits.

How to join pits when i am low lvl?

You can't. If you want to join higher pits, you have to be 60. Unless you have someone that can help you. If you're talking about solo leveling from start, you can just do the seasonal and make it to max lvl in no time. Literally just play the game and you'll level.