American attending first united match at Old Trafford

[BK United]

Hello my United Brothers and Sisters,

I blessed to being going to Manchester in August and want to attend the newcastle match at Old Trafford.

Couple of question for us Americans wanting to attend a match

What is the best website to a buy tickets?

How much can i expect to pay for a ticket?

what is the best sections to sit at in Old Trafford?

Thanks