Things to check :
You have a key - are you sure it’s the right key :?: I.E. have you personally used that key in the past :?:
If you can answer yes, then two options present, 1 the key is damaged, 2 the lock is damaged.
If you answer no, then add a third, you don’t have the right key …
Some nice clear pictures of the key would help.
If any of the above applies your best bet is going to be to call a professional locksmith to look at the problem for you.
Now, if the key is correct you would insert it, probably with the bit or steps in the lower part of the keyhole and turn it, it will only turn approx ⅓ rd of a rotation and then withdraw (probably in the upper part of the keyhole). The turn the brass handle on the door. Done.