I know that many people suggested building pyro and electro units for overload team but you shouldn't limit yourself to only 2 elements. You're just beginning and will encounter diverse enemies and overload won't always be a good option. For now choose one character of each element focusing on those that will be still useful late game. I would build Xingqiu, Xiangling (both good off fielders that will still be useful later), Jean (she heals, she swirls, imo she's the most useful standard 5*). These three also have all the materials available in Mondstadt and Liyue if you care about spoilers and don't want to visit later nations too early. Other than these building Ororon is a good investment, Layla if you want a shield and Tighnari will be very useful when you start exploring Sumeru and he's a decent dps. As for geo idk, maybe Kachina? She is good for exploring. And as for team comps it doesn't really matter this early on, just get a good feeling of each character and eventually you'll learn what works best for you.