See that you already made you purchase so congrats! All these guys are right on. The megapixels won't really matter unless your doing large prints. The 5100 will easily allow you to print 20"x30" with stunning quality. My first DLSR the D40 was only 6mp and still had great images, a good sized sensor is the biggest factor a lot of the times from point and shoot to DSLR, not so much MP.
The Flying Dutchman makes a good point, get some good glass. The lower the f/ stop the better. 1.2 is a little crazy awesome something to be jealous about, but I would suggest checking out a 35mm f1.8 or 50mm 1.8. Relatively cheap prime lens that are really fast and give great Depth of Field. When you do get lenses, jyst make sure they are AF-S, AF lenses will not auto focus on the 5100.
If you have any questions feel free to message me. Love photography