If i start the expansion now would it take me weeks to be ready for pvp because its i lvl now? how does it work?

This really depends on what level you currently are, and how much time you have to play. Leveling from 100-110 doesn't take horribly long, but opening up all the world quests can take some time. If you are going to get back into things, here is a good gearing guide that also covers how Legion gear works: http://www.wowhead.com/legion-gearing

Enjoy!

If you have friends that can help you once you reach level 110, it can go really fast

its insanely fast with mythic+

with mythic+ and artifact knowledge catchup in 7.1.5, it wont take long

The catchup only works if you already have a character with higher level artifact knowledge and if your not in a guild getting anyone to even take you into mythic+ will be a miracle.

Even if you don't have another character, you can buy lvl 15 AK for 7500 resources. And as far as mythic+ goes, yeah without friends or guild mates it can be a little rough but you can just make groups "my key your carry" and hope you get dht, maw, or cos for your rank 6-10. And world quests and shit make it so easy to get mythic ready. If you want to play the expac, the catchup mechanics are out now.