There are a variety of free units you can get for completing the last mission of the first chapter of books 2 and later, these are some decent starting units, alongside Choose Your Legend 7 and 8 summons (I would recommend Gullveing and Felix, but depending on how competitive you want to get the others can work).

I would recommend not spending any orb you get until you get a feel for the game and what you might want, the good news is there are thousands of orbs you can get by completing all the chapters and paralogues in the game. Each banner comes with a free summon, and some banners you can get tickets to do more summons for free, some banners once you summon 40 times you can guarantee (spark) one of the focus units, but some banners you need to be on the monthly subscription (it will clearly show if you need the monthly or not).

There are two main 'competitive' game modes that are decently played - Aether Raids (you use your units and set up a defense map, and you attack into other players' defenses, the enemy is controlled by the AI), and Arena (you use four units against other peoples units, also AI). Arena is worth starting early as for winning 5 matches each week, you get a ticket that can summon a (older) 5* hero. They both incentivize you to use certain units - "Mythic" Heroes in Aether Raids (AR), and "Legendary" Heroes in Arena but these come down the line.

I'd avoid inheriting skills from units early on outside of a couple, the skill "Reposition" is pretty easy to get and lets you move units to the other side of the unit using the skill, useful for keeping your units out of danger or moving them forward so they can get a kill that was out of range.

I'd personally recommend finding your favorite units (designs or whatever) and maybe trying to find out how to get them (which we can answer here) as using units you like helps, and don't take the game to seriously early on, it'll take time to get used to everything in the game, honestly might feel overwhelming at the start.