First of all "aimer" is the verb "to eat" in the infinitive form..you have to choose a tense for you verb .. if it's the present you must say "je aime" ..but in french when there's two voyels jE Aime we delete the first one and replace it with an apostophe---> J'aime ..
Now , let's talk about pizza ... you have to add the appropriate article before ...
Finally our sentence will be : J'aime manger de la pizza .