Hi Ozgood. We were all newbies once.
On the runDisney website for the 2017 Marathon weekend, click the "Runner Info" section and choose the Race Waiver link. It will bounce you down to a link that you have to type in your surname and DOB. It will return a link to a PDF to print out and bring with you to the Expo to pick up your bib. It's the standard boilerplate that if something happens to you while you are on the run, Disney cannot be held responsible. The waiver will have your bib number at the bottom.
Then go back to the Runner Info section and click the Corrals section. It will bring you to a PDF link for the Half that will show what bib numbers are in which Corrals (lettered A through P). Then if you are REALLY into it, in the Runner Info section choose the Official Guide link and open it and go to the page for the Half. It will show the course map, corral placement, and Finish Line/Award layout. The corrals are started every 4-5 minutes.
FWIW it looks from the course map that the Half will NOT be running through the castle (the Full appears like it will). I've heard that the castle thruway can be a bit of a bottle neck.
Good luck with your Half, Ozgood. I did it most recently in Jan2015 and it was a good experience.
Bama Ed