To log in to Matchday you need to ensure have one of the required roles for access, which will most commonly be a team manager/coach, player/parent or club treasurer or secretary.
You then need to enter your email address and connected password, which should then get you logged in. This combination of details will be the same ones that allow you to log in to myaccount.thefa.com or thefa.com.
You also have the option to go through the Forgotten Password or Sign In Using Fan flows which can help get you in without needing to reach out to your club or for FA support.