GEARING YOURSELF
Take the commendation rewards from every class mission. Buy a full Trainee set (armor + weapon + offhand) from the new Level 7 Basic vendor on Fleet when you have enough comms.
Upgrade your Trainee gear at the Basic comm vendors on Fleet at levels 13, 29, and 41. Every three class missions under 12x will give you enough comms for a full refresh on four pieces of gear. You don't need to upgrade every 4 levels; every 8 or 16 levels should be often enough, especially if you're using a healing companion.
Head to Makeb as soon as you hit 47 to reach 55 in the shortest time. Your level 41 set should get you through to 47, and after that the bolster on Makeb will carry you from 47 to 55. As soon as you hit 55, play through [SOLO] Forged Alliances (also bolstered), which will give your first set of (162 rated) SoR leveling gear.
Continue on and play through Rishi, Yavin, and Ziost, and you'll wind up with a full set of blue 190 rated gear by the end of Ziost. The leveling sets from Yavin will work fine for Ziost, so just ignore the 186 and 192 comm vendors; all of their kit is now obsolete.
Pause anywhere between 56 and 60 to pick up some cheap Ruusan relics off the GTN. The greens are almost as good as the purples and cheaper, but the purples are usually affordable anyway. You could pick up a cheap earpiece and implants as well. All these slots can be left empty up to 60 if you like.
As /u/bstr413 [1] pointed out: Don't forget your color crystals after level 55! All level 56 and above gear comes with empty color crystal slots. For an economical slot filler, just grab the cheapest cartel market crystals you can find on the GTN in your preferred stats, then unlock in collections account-wide for a few CC, and you'll have an endless supply of crystals to fill out all of your 55-60 gear. If you got the recent sub promo crystals, those also work (not sure if they're collectible).
NOTE: The 12x bonus ENDS as soon as you hit level 55, regardless where you are at in the storyline. At that point you should move on to SoR content to continue leveling and gearing at best speed. You can circle back later, once you hit 60 and have your Ziost gear, to finish any class missions you left behind.
NOTE: As /u/villige_idiot [2] pointed out, the comm vendors on the planets did not get the new reduced comm pricing when 3.2 arrived, so always do your shopping on Fleet for the best prices.
GEARING COMPANIONS
For your first 12x playthrough, throw some cheap orange cartel market armor shells (or one of the free legacy bound armor sets, if you have them) and an orange weapon on your companion. Either hand down mods to them as you level, or upgrade the companion as well if you have enough comms. This works for all humanoid companions, but sadly not for droids, which can't wear regular armor.
The Level 7 Trainee gear is class locked, so companions cannot use it. But you can get orange offhands for all companions from the Nar Shadda comm vendor (requires level 20-something).
As you level up and upgrade your own gear, pull the mods out and put them into legacy armor shells to use on companions. Once you have a legacy set for each main stat at levels 13, 29, and 41, you will be covered for every companion in the game except the droids, for all future alts.
At level 60, you can get full sets of 192 rated legacy bound armor and weapons for all your companions, from the [WEEKLY] on Yavin IV. There also is a [DAILY] quest on Rishi that gives 192 rated legacy bound companion offhands. This gear is useable from 55 and still quite good at 60.
EDIT: Glad to see people find this helpful. And thanks for the gold!
