Large Prints, where do you get yours?

Novice photographer here. I've been using my Canon T2i for about two years now. During that time I've managed to take a few decent shots. I want to get some large prints. Maybe 11" x 14" or larger, up to movie poster size. I've only used Walmart for small 4x6 prints because they're cheap enough, but these larger prints I would want to look really nice so they can be framed and take up some sizable wall space.

Where is a good place to order them from? My camera takes pictures 18.7 megapixels. If the noise isn't too bad, I'm wondering what the largest print I could get before the lack of detail becomes obvious.

Also, I need some advice on editing my photos for fixing things like saturation and contract. I have access to Photoshop at work, but would it make sense to get something like Lightroom? My monitors are 5 year old Samsung TFTs, and I don't think they are exactly wysiwyg when it comes to photos. Not sure if there is a way to calibrate them either.