The first step in the process is creating an account on your local ticket reseller's website. The Tokyo 2020 homepage does a good job of listing out partners.
If you are applying from America then you should make an account on CoSport. From there you’ll get a log-in and password and will be able to request tickets to purchase.
On the site, you can search for events by sport, date, if it is a medaling session, etc. In total, you can request 30 tickets. It’s amazing how quickly you reach 30 tickets -- I submitted for nine different events including the Opening Ceremony, football, basketball, athletics, judo, swimming, gymnastics, fencing, and baseball. We’ll see how many I actually get selected for.