As someone already said, don't delete your old characters, especially your oldest one: birthday gifts are per character rather than per account. You probably have a ton in your mail if you haven't gotten them yet. Probably best to be near a back when you open them; they'll take a lot of inventory space.
It's up to you whether you want to dive back in on an old character, or start from scratch again on a new one. The nice thing is that leveling is pretty quick, and the game is very alt-friendly; you're almost expected to have multiple characters, and endgame equipment is all account bound rather than character bound, so you can share it around between characters.
I'd use the 1-80 journey to figure out what you enjoy in game. You get XP for all of it, so once You've checked out all. (or maybe most) of it, do the bits you like. Personally,I like exploration and story, so I do that bits. Also instanced PvE. There's also WvW, PvP, races, adventures, crafting, collections, achievement hunting... all kinds of stuff.
Enjoy it, and welcome back!