Let's use the word manger (to eat) as an example, the sentence will be I have eaten
First: let's get the pronoun 'I':
je
Next, we take the auxillary verb (avoir- have)
ai
then we take the infintive of the verb :manger, remove the e and replace it with an é and we have the verb :
mangé
now we join the 3 words to make a sentence
je ai mangé
but in french, two vowels (e and a) cannot be together so we use an apstrophe in the place of the vowel which looks like this
j'ai mangé