Take a slow deep breath in then out. Repeat. Continue to repeat until your blood pressure drops.
If you're staying offsite or booking onsite with a room only reservation then it is cheapest to buy tickets in advance from
Undercover Tourist using a link in the Mousesavers newsletter.
UT is extremely reputable and is an authorized ticket seller. They will mail the tickets to you.
I specified offsite or room only reservation because if you've booked a WDW package then you have to purchase your tickets as part of your package and you will receive them when you check into your resort. They will be electronically coded to your KTTW (Key To The World) card which is also your room key.
I keep my tickets in my jewelry box. You just need to come up with a place that you know you can remember and then make sure others know where you've put them just incase you do forget. I have a spreadsheet with all sorts of planning worksheets, one of which is a packing list. Tickets are an item on the packing list. I do NOT cross off anything on my packing list until it is actually IN a carryon or suitcase, even if I know I have it put aside for the trip. This way I don't forget anything. Also, if I'm worried about forgetting where I've put things like tickets then the bottom of the packing list is where I put notes about where I have stuff for trip. By writing it down I don't need to worry about forgetting.