"I eat a fruit" is grammatically incorrect. "fruit" is only countable when you are talking about the definition "an end result" (i.e. a fruit of one's labor).
Similarly enough, "fruits" when talking about them in a botanical sense is rarely used. It is uncountable and normally used in the singular. In fact, in this very sentence, "fruit" can either be a mass noun or plural. "a piece of fruit" is normally what we use for the singular. :) (I see you are from France, fruit behaves a lot like "raisin/grain de raisin" in French.
That is the conjugation of verb "yemek" or to eat
Ben yerim --> I eat
Sen yersin --> You eat
O yer --> He/She/It eats
Biz yeriz --> We eat
Siz yersiniz --> You (plural or formal) eat, it is like y'all or vous in French
Onlar yerler --> They eat - but you can also use Onlar yer and that's more common I guess than Onlar yerler according to my Turkish friend says :)
I hope this helps :)