Help with school database

Hi there everyone, I have to do a bit of work in school and we have to build our own database. I need a bit of assistance as I've ran into bother.

I have to create an invoice based system. Here's a few screenshots of what I have at the minute.

Here's a list of the tables I have at the minute. Can you check if these are right? Do I need a key field for Line Items?

http://i.imgur.com/zzz8t.png

I have setup a report to try and get an invoice to work, but not sure how to group the items together. They appear as separate invoices if you get what I mean.

http://i.imgur.com/Uqo1O.png

Could anyone suggest any ideas? This is mickey mouse stuff for you guys!

Thanks