Look at the previous post about the tier lists. If you manage to pull any of the high tiers, check their ivs (red bad, blue good) and don't level them until you can merge out bad ivs or else youll be raising it on shit stats that you cant salvage later.
If you got the good IVs, raising it to lvl 40 is only the beginning to making it the best it can be.
You can merge clones of this unit but make sure they're all the same rarity and you can do this ten times. You can use feathers to make 4 stars into 5 stars. DO NOT merge a bunch of 4 stars thinking they'll retain this if you eventually raise it to 5 stars. You have to merge it at equal rarity. This will take a long time, unless you dump your entire load of orbs on a banner which i think is what most vets do.
After that flowers will raise the unit to it's max potential but it'll take a while. Otherwise only summoner and unit supports will raise its stats I believe.
Save orbs. 100 is not a lot.
There's a lot of ways to get resources when you're starting out, you just have to try all the various modes and quests and not blow it on meme units.
Unless you need feathers, i think its also pretty much better in every situation to not send home units if you need space but turn them into combat manuals. This way you can save barrack room and save the units for their precious skills by converting them to manuals.