All that Ade said
An féidir liom...
Or
An bhfeadfainn ...
After the introduction you can say if appropriate
Céard/Cad atá uait? When said in the right context and tone of voice I don't think it's rude.
But if you said it as soon as somebody walks in the door then like in English (What do you want?) then it wouldn't sound well.
Another option:
Ar mhaith leat cabhair?If showing somebody something like in a shop:
Ar mhaith leat é seo?An dtaithnaíonn sé seo leat?Like in English there are loads of different ways to phrase it.