There are two kinds of tournaments: (1) Sanctioned by USA Fencing Academy (USFA) and (2) Not sanctioned by USFA.

In order to fence competitively, you will need to purchase competitive fencing membership with USFA (https://www.usafencing.org/) for an $85/year fee. You will be able to search for tournaments held by USFA on their website and register for them 

For unsanctioned tournaments, the primary site is https://askfred.net/ 

I highly recommend you to participate in as many tournaments as you can since no amount of practice can match the real bout experience. Also, USFA has a rating system where a fencer starts as U (unrated) and can progress to E, D, C, B, A in that order where "A" is the top rating. The ratings are associated with the year in which they were earned. For e.g., my rating is E2020. These ratings are valid for 5 years. Ratings are achieved based on your placement in an event and the event classification. The event classification depends on

(1) the minimum number of competitors

(2) the number of rated fencers required

(3) the number of rated fencers that need to finish in the top 8 or 12 depending on the event type

The reason for #3 is to ensure that the fencers don't throw their match to let someone else win.

The event classification chart can be found here https://member.usafencing.org/files/ClassificationChart.pdf