New player, just completed all the campaigns, now what? So many questions

Should I jump straight into raids/dungeons? Is there a particular order I should do them in?

Should I spend time getting my power level up? If so what’s the most efficient way to do that?

Is there a meta warlock build I should be working towards? Meta weapons/armor I should be trying to get? Is the build that I should use dependent on the particular activity I’m doing?

Also I’m a solo player, so how do I find people to do raids/dungeons with? Is there a website or something that I should use?