How to select first item in list

I need to select the first item in a list of items with every new iteration. The items will always have a "Buy" and "Sell" checkbox next to them but the length of the list and the items therein will differ from iteration to iteration. I need to just "check" "Buy" for the first item in the list every time. I figure i need to use JS for this but I am not sure how exactly. I don't think it is necessary to get the length of the list if I am going to use always the first item.