Welcome to the forum. Some general basics on defense team as quoted from Coppersky’s Compendium:
One team must be marked defence. The team marked defence is what gets attacked when you are raided. The positioning of your heroes matters in your defense team. When first starting out put your strongest dead centre & weakest at the edges. (New players - put your Bane front and center on your defense team! He’s the best you have for taking the hits!) When you are being raided, the centre gets hit most in attacks, the far sides are hardest to hit. When you get a stronger team you’ll probably put strongest dead centre & heavy hitters far sides with healers or weaker heroes in the other 2 spots. On your defense team choose your strongest heroes/best combination of special skills. Your defense team does not depend on gem matches to hurt opponents, so you do not have to worry about having one of each colour."
That said, I would focus on 4-stars for defense. 3-stars will die very quickly vs 4-stars, and 5-stars will take forever to level - forever and a day if you have no mats yet. Given ypur current roster i would set up your defense team as follows (focus on levelling them):
Left Wing: Caedmon - dispels attacker buffs before any of your heroes fire. Replace with Sonya if you get her so no hero will be the same color as Buddy.
Left Flank: Rigard - your healer and cleanser.
Tank: Buddy - probably the best 4-star tank IMHO. Tough, debuffs attackers, and generates minions.
Right Flank: Hu Tao - slow speed, but is tough and should be able to fire in Flank position. Does AOE damage and adds another opppnent debuff. However if you get Li Xiu I would run her here instead.
Right Wing: Kelile fast sniper.