Focus unlocking your favorite Star Wars characters lore wise and build teams around them.
If you just want the most powerful characters, you can google it.
Best ways to grind crystals is completing dailies and events.
You earn XP the more you play the game. XP can't be gotten faster unless you buy the HDB. (refreshes help but overall doesn't make enough difference to matter)
Guilds usually recruit based on GP, GLs, how far in the game you are, how active you are, whether you have discord, and some other specific factors unique to the guild. There is a place in the forums you can post when looking for a guild.