If you have the whole set of goblin town then you could have the goblins against rohan, just to get used to the basic army v army game. Then once you are used to the basics, include a hero on each side (gandalf / goblin king) and try your hand at some basic terrain (hills, rivers, low walls) that you could make out of card.
My very first few games I played were the old ROTK boxset and I remember I just ignored the points values per model, just put 'good v bad' and me & my brother had fun just learning the basics. Once we got used to that, we added in other rules in each battle, bit by bit.
