You can start here:
You will need to connect your MBP with your Apple ID. Best to start as new, may cost a small amount of money, but it will eliminate any possible issues further down the road. The things that you are looking to update are attached to the previous owner's ID you need to make them yours.
See the following to make this mac yours, you mat have to spend a little money, but think of how much it is worth over the hassles over time.
The previous owner obviously did not do this.
So are you suggesting that I back up everything on my MBP to an external hard drive and then completely restart the computer to its out-of-the-box state? Will it remember all of my applications?
Yeah the guy literally gave the computer to me as if he had just used it. The Mail app was stilll signed in to his account. I wish he would have wiped the computer, but I didn't think about it till later. Ugh, I wish I jad just started from scratch before it was too late to do this whole process.