Sadly, they don't have all of S1 up at this time. So many of the beginner foundational episodes where you get to know the brothers aren't up. But many of the mid-late season episodes are there and that is when S1 got REALLY GOOD.
They do have the episode called "Faith" up right now. if there is any episode I would urge you to watch, it's that one. All the episodes they have up are good and omg "Shadow"! Meg! John! demons! But "Faith" is the episode that snagged me from casual viewer to getting excited and making icons and stuff. I think it's one of the series best. And it's my second fave episode ever. In hindsight, it's very enlightening if you are watching S4 and S5. Not only does it deal with Sam's and Dean's faith vs lack of faith, but also Sam's drive and Dean's self-doubt. There are even some nice parallels to trying to save Dean from S3 as well. Very awesome.
Here's the episode: While on a hunt, Dean accidentally is electrocuted and permanently damages his heart. (It's always that poor boy's heart.) Sam takes Dean to a faith healer hoping to save him, but they find out there's more going on than it first appears.
If you don't want to watch the whole episode - WHY NOT?! - here a couple of clips to show you what the episode is like. I tried to embed them, but I seem to be having some trouble with TNT's embed code with LJ.
Dean and the faith healer:
Probably the best use of a classic rock song in this show ever.
"Don't Fear the Reaper."
I could quote this episode all day, but I've always liked this bit:
Roy: "You've come here to be healed, haven't you?"
Dean: "Well, yeah, but maybe you should just pick someone else."
Roy: "I didn't pick you, Dean. The Lord did."
ETA: Another good exchange I love and makes so much more sense now:
Dean: Why? Why me? Out of all the sick people why save me?
Roy: Well, like I said before, the Lord guides me. I looked into your heart and you just stood out from all the rest.
Dean: What did you see in my heart?
Roy: A young man with an important purpose - a job to do - and it isn't finished.
If you have some time, go watch and enjoy :)