My tyre says 700x38 on the side of it. what size inner tube would I need for the wheel? I've ordered a 700x35 inner tube but it seems a little too big for it?
That will be fine.
38 is the measure of your tyre width. Most inner tubes can be used in a range of tyre widths eg 38-43 (it should say on the box) They all seem a bit big and flappy until the tyre is on and it's pumped up.
Be careful not to pinch any of your inner tube between the edge of the rim and the bead of the tyre.