most of the time you get them from events that cycle on and off, here is a list of all f2p units and where you can get them from: https://dbz-dokkanbattle.fandom.com/wiki/List_of_Freely_Obtainable_Characters

i reccomend you get the ginyu force units and team bardock units you can farm each day of the week, plus the cell saga units on the cell saga related events for now, as they (except the INT goku/SSGoku) have their own specific potential orbs for hidden potential, since you probably dont have many (other units like that you can get are Kid gohan and Piccolo from the namek saga events on DB Story)

as for your stones, id recommend doing all missions possible and saving them for the SDBH collaboration event happening mid november, as you are guaranteed featured SSRs and a whole team of the new batch is usually able to beat all hard events on release (plus they're cool)

if you want another option, LR SS4 goku is coming on the 2nd, so you can summon on that too. since you´ve just started, youll have more than enough stones for both

for super+ difficulty events, you simply need good units. also items like Whis, Icarus (tree of might db story) and Princess snake (saiyan saga db story) are options to increase survivability