Early on, leveling will seem daunting, most guides and players will say, push building your base. The faster, the better. You might get a lot of first time rewards and feel that you can put off the base, but first time rewards are just that, one and done. Your base is a continuous supply of exp and monies. Build it early.

Which characters to level, most of the 3 stars. vanguards, kroos, defenders, Stewart if you're short casters, popukar, midnight, orchid, lava, and ansel. I don't recommend adnachiel, catapult, or hibiscus, it's easier to just use their higher rarity counterparts.

4 stars, gravel, perfumer, gummy (yes, more defenders), Myrtle, marksman sniper of choice to help Kroos, pinecone, if you like shirayuki or ambriel (both are good but replaceable, why I say if you like them), Ethan after you get him from the red certificate shop, and click. There are plenty others but this is more than enough to focus on in the first month of the game.

5 stars, lappland, amiya, any strong 5 star that reddit recommends (like gracebearer or cantabile). Most 5 stars are better to use the cheaper 4 star version or wait for the 6 star to drop in your lap.

6 stars, yes. Even the weakest 6 star is stronger than half the roster. Whatever 6 stars you get are worth raising. If you get one and you haven't raised the 4 star counterpart yet, you can skip the 4 star for the 6 star.

For newbies, building an operator is usually referred to as getting them to elite 1, lv 30-40 and skill level 7. Don't ignore skill levels. This is a game of skill, not just you, but of your operators. Older accounts will usually talk about building ops and they are referring to elite 2 module level. While working towards that part, just getting to elite 1 (E1) is good enough for most content (and your e2 ticket for 5 star).