Fit can be used in so MANY different ways. In most cases, it is used as an adjective.
Fit - to be the right shape/size/number of
Because I am fit, I will be able to go through the door
(Because I am this size, I can fit through the door)
It is uncommon/rare to see it as a noun by itself, but it does have a noun form
fitness - the process of getting fit
Ex: I read a book on fitness
Lastly, you CAN use it as a verb, but also not common.
Ex: I was trying to fit myself into the t-shirt, but it did not work.
(I was trying to be (the right shape) of the shirt, but I am not the right shape)