Welcome to the Star Realms rabbit hole … it’ll take a while to climb out, if you ever do.
I wouldn’t start with any expansions–get comfortable with the base game. Once you do, I’d lean toward adding smaller expansions that tweak the game slightly, rather than the bigger ones (Colony Wars & Frontiers) that contain new base sets. Love those sets; just wouldn’t start there.
My favorite small expansions are Bases & Battleships or Fleets & Fortresses, but YMMV. I would not recommend diving into Gambits or Missions early on.
For advice … there’s a ton of stuff on Megahaulin (https://megahaulin.wordpress.com/), but it can be tough to navigate all of it. The Star Realms Fan Created Facebook page has a lot of experienced players who are happy to answer questions. And we’ve got a few experienced players around here as well.