Dont know how to start and build my team

You should keep iskandar in your front slot then put Tamamo Caster with them, then you should put Arjuna in the back, you’ll want to use Iskandar as a tank and Tamamo caster as a helper, then use Arjuna as a backup.

Higher stars does mean higher base stats but not necessarily the best skills-set or team synergy. Because Class Advantage is extremely strong generally you'll want to edit your team for every fight so when just starting out your best option is to get at least one of each different class of servant and a couple of solid supports to provide buffs.

Start out by leveling Iskander and Arjuna, they'll cover you on the Archer and Rider front. Berserkers like Tamamo Cat are great for easier quests or farming since they have 50% bonus damage against all enemy classes but they die very quickly.

So you still need a Saber, Lancer, Assassin and a Caster.

For Saber Caesar and Fergus are both solid. Caesar is generally considered better overall since he provides a lot of good team buffs but Fergus is stronger as a solo unit, so take your pick.

For Lancers Diarmuid and Benkei are pretty bad. Leonidas is a very good tank/support but that's not really what you want to level first as a new player. Keep doing your daily friend-point Summons until you find Cu Chulian (Lancer), who is hands down the best 3* servant in the game and level him instead. If the RNG gods hate you and don't want to give you him, Cu Chulian (Prototype) or Romulus are decent options.

For Assassins Hassan of the Hundred Personas is quite solid so I'd put your assassin Embers into her.

For an offensive Caster you'll want to level Medea.

Once you have all of the 7 basic classes covered you'll want to look into rounding out your team compositions. Pickup Hassan of the Cursed Arm for a Critical Star generator and Hans Christian Anderson from Friend Point Summons for team buffs/support.

Oh and don't neglect Mash. She's easily one of the the best defensive supports in the game once you get though most of the story quests and Camalot to unlock her skills and upgrades.