1) You don't need to put something on the inside if you don't want to. Most clothing doesn't because it makes the fabric really heavy to wear. Take a look at your jackets/sweaters. If you want an interior color then it would be a different story.
2) Umm... it could be Decor Bond, it looks like this: http://ecx.images-amazon.com/images/...HL._SY300_.jpg
here is a good article about what it's for: http://www.sewsweetness.com/2012/05/...terfacing.html
The other thing it could be is normal semi transparent pattern paper from when people make their own patterns. Other than that I'm not sure.
3) Yes. You should wash your fabric before sewing it. Fabric tends to shrink a little. Best to get it out of the way, you can cut your fabric and then wash but it's faster to just wash your fabric when it's not cut in case you end up making something else with the fabric, it's already been washed.
*I just started making my own stuff recently so I'm also learning a lot. I took a beginners fashion class during my time in art school (I needed a low level elective >_> ). I can't help a lot but if you have any questions I'd be happy to try to help and learn with you :3.