I am level 20 ive done the first Jarl quest where you kill a forsworn leader at a camp, and ive done alot of bounties for the steward but it still doesnt give me the option to buy the house. I know the next Jarl quest is the one where you get a shield for him but what do i need to do to start the quest? Are there any quests in Markarth that i need to do first?

If I remember correctly, you had to complete the 5/5 "help the citizens" of Markarth quest. I gave a coin to the "obviously homeless" guy in the Tavern on the main level. Silver Blood Tavern maybe? Also there's a Thalmor just before you reach the Jarl, that'll count as one well. You mentioned the Forsworn/bandits quest, I'm not sure if that's how I got the other three. Hope this helps as it's all I can recall.

I didn't get the option to buy it until after I got back from retrieving the shield - then I was good to go. You don't need to do the "help the citizens" bit to buy the house, that's for something else.

ETA: I should point out that I wasn't able to do any more quests for the Jarl once I'd completed the Forsworn Conspiracy - the option to ask him about work just disappeared and I wasn't able to buy the house, I had to go back to an earlier save and do the Jarl's quests first - I guess that's a bug.

Wow i just did the Dibella quest and i finally got the quest for the shield. Ive done the shield quest and got the option to buy the house. But or the Thane quests where you have to help 5 people, if youve already helped 1 or 2 of them does that count towards the 5?

Yes it counts - when I started that quest the objective that required you to help 5 people auto-completed while I was still talking to the Jarl

Ok cool thanks.