Gab - lots to know, and many on here can give you great advice!
Check-in can happen in a couple of different ways...
You can check-in online prior to your arrival and, depending on your time of arrival, you might receive a text with your site, allowing you to go straight there upon arrival. There seems to be some lack of consistency on this, but it does happen sometimes. Others, especially those who have a strong preference in site location, plan an early morning arrival and check in upon that arrival in order to try to influence their site assignment.
If you don't go straight to your site (because you didn't get a text or you haven't done on-line check-in), you may be asked to park and go inside to check in (if you are particularly early or late). Otherwise, the actual check-in will happen at a pull-through booth where everything will be taken care of without the need to park, get out, etc...
After check-in, you will proceed to your site. If there is any issue with your site (you don't have enough room to back in, you were hoping for something different, etc...) there is a phone number they provide you that allows you to call up to the front desk and asked to be re-assigned - avoiding the need to trek back up front, park, etc...
As for magicbands, they will automatically activate once you are checked-in and assigned to a site.
There are many on here willing to help. Some (like me) were in your shoes as a first-timer very recently. Others have been around forever and know the Fort like their own homes. Feel free to ask and we will all kick in.