So I just started and I am not sure which characters too use?

Heracles should be the central focus of your team. He packs immense power and surprising durability for a Berserker. Even though he’s a 4* servant, many players treat him as though he were a 5* servant; that’s how good he is.

I would suggest using Mash and Caesar along with Heracles. Mash helps boost his defense while Caesar helps boost his offense; interchange who starts on the frontline based on whether you need more defense or more offense. Early in the game, a power offense strategy should be very effective.

I’d definitely have Cu as your primary rearguard. Cu’s claim to fame is his survivability. He’s not a power punching unit, but he’ll save you if you get in a pinch!

Fergus could help Heracles get more Buster chains, but his skills aren’t terribly useful, for the most part.

For the servants that you have eight now I would say focus around Hercules. In terms of raw damage he is one of the better 4* Beserkers and commonly used at high tier for his damage output for clearing ads easily, I'd pair him with cú or ceaser (cú being for sheer survivability with protection from arrows if it seems a mission may take awhile) other than that is say pair him with mashu and Caesar for the damage plus that career offers and the defense boosts that mashu offers.
Hope this was explained well enough and good luck to you~

Herc is the best servant you have by far right now, it will be solid to set him up with Mash to keep him alive while he kills everyone. Caeser is the most solid 3* saber available right now from what I understand, so he'll be good to invest in. Fergus and Babbage are not that powerful, so I'd not put a very high priority on them. Cu is your backline because his survivability is pretty crazy, he's solid to invest in for sure.

In addition to what has already been said, you should make sure to roll the Friend Point banner every day so you can get five coppies of the two best Bronze servants, Hans Christian Anderson and Hassan of the Cursed Arm. Hans is a top quality support and Hassan is the best budget critical star generator in the game.