Hi Rencall, I would suggest picking a server with a guild that has other returning players And isn't against boxing. Then do you have any resources to work with (i.e. any higher level toons)? Are you wanting to play the group from 1 to 115? Or, add into characters you have to make a 6 box?... I'm asking because the path will be different.
From the short time that I have been doing this: If you have a trinity (tank, heal, CC) then add in what you like, your 6 box will be strong enough. I'm sure others will comment, too (in case my way isn't the best). If you are playing all the way up. Hit as many achievements as you can along the way. When you get to Seed of Destruction (and beyond) you can hit the Mercanry and Partisan quest lines (in Achievements). Once you get to Underfoot those Merc and Part Quests put you in a good place to get General Achievements done - and those give you things that are useful, fun, and make you a badass. Keep an eye on your Achievement Rewards from then going forward and you can tailor what you do and where you go around the best rewards.
Past that hit content, you liked from before. If you didn't like some content go look and try the new stuff.
I don't think there is a wrong way to do it... because only a fraction of the game is under level 115 (based on AA's gained by 115 compared to maxing your AAs). Plus, back to the guild suggestion... if they raid (even once in a while) you can build up better gear once you reach 115. That makes the content easier and you can always go back and finish/do/redo lower-level content too.