i need help building a team

It seems like you are playing on the jp server. For now, don't worry about team comps. Use class advantage to get through the story. Try using helena, chevalier, heracles, and mash. Perhaps the jp veterans can help you out.

How far along are you in the story? For now, I would recommend focusing on clearing it as far as you can using a strong berserker friend support (or whatever is class effective in each battle), while also leveling your servants on the way.

Since buster teams are still top tier, you should try and level up your Heracles (one of the strongest berserkers in the game), as he will do wonders killing enemies with his kit.

Caster Gilgamesh is a pretty good pick as a support, but he is more focused on Arts based teams, so there isn't too much synnergy between him and Heracles. Still a very good servant to use though.

Helena would synnergize better with Heracles, as she has buster up and NP charge skills, which are very useful especially with Herc's low NP generation.

However, the best support servant in my opinion would be 2* Hans Christian Andersen with a few skill level-ups, as he can charge team NP, his own NP, and in turn boost attack, defense, and HP regen for your team. I would level him up and keep him as my support until you can get your hands on either Waver or Merlin.

Between Chevalier and Suzuka, Suzuka is the better choice for a saber, her kit and stats are just overall stronger.

I wouldn't bother leveling Fionn as your lancer yet, and would instead go for the 3* Cu Chulainn lancer (not prototype), as he is insanely good for surviving in long fights, while also packing a punch with his NP. He is the best lancer you can level right now, especially since you can quite easily get him to high NP levels thanks to his low rarity.

Also, Mash is one of the best support servants you can have for defense, so having her leveled up either as your frontline support or in your backline as backup is a very good idea (especially since her cost is 0, so she is basically free to use)

Here are some example teams I put together with your servants:

Buster team to speed clear story quests (also your go-to team for the moment):
Front line: Heracles, Hans or Helena, Friend list Berserker
Back line: Mash + whichever servants you want to use, as long as they are decently leveled

Another buster team (very useful if you want to completely obliterate a single boss without having an NP charge Craft Essence):
Frontline: Heracles, Helena, Support Waver/Merlin (preffered Merlin)

Arts team (good synnergy between Helena + Gilgamesh + Fionn):
Front line: Helena, Caster Gil, and Support Tamamo no Mae/Waver
Backline: Hans, Mash, Fionn

I don't think that quick teams are really viable with what you've got, and they're not very useful for new players, so no point in making one.

Hope this helped, if you have any more questions, don't hesitate to ask :D