Well First off. Do a lot of heroics. They will boost your level quite significantly making much stronger. They will also give you some decent gear as well as quite a bit of credits. They are probably the best way but if you don't like to grind consider doing some Flashpoints as they also give you great gear, credits as well as exp. They are a fun way to level up with friends or with randoms! Also you could but Modifications to make your gear even stronger. you could use the credits you make from heroics to buy mods to make your gear even more legendary. Also consider joining a guild, they will give you a great exp bonus as well as most good guilds will help you out in missions and help you get back up to date. I would recommend joining the Vindicated Empire if you're on BC server as they will be able to help you quite a bit with levelling up and will do missions with you. They will also teach you anything you need to know and are just great friendly guild that is here to help you.
If you Wish to contact the guild or join the Vindicated Empire- Join their Discord https://discord.gg/hW2uPwD\
or contact Ronos Vett in game, he will help you