The common definition of "I eat" is "saya makan". People usually say "saya SEDANG makan" for the translation of "I am eating". The word "sedang" indicates the event is being done (the moment of speaking).
Long answer (this is a brief explanation about Indonesian grammar in general):
In English, to indicate different time when the event or action is done, we use different tenses.
For example, you are talking about "makan" (to eat) as an action you do every day, 3 times a day. You use present simple there, and the sentence will be "I EAT (every day)". If you are talking about "makan" (to eat) as an action you are doing right now, the moment you are talking to me, the moment you are speaking, you will be using present continuous there, and the sentence will be "I AM EATING (right now)". That is how things work in English.
In Indonesian, things do not really work that way. The word "makan" will still be that way no matter when it happens (what tense it should be in English).
There is no formula where "makan" needs to be "makan-s" (as in present simple), or "to be makan-ing" (as in present continuous), or anything like that (TECHNICALLY, GRAMMATICALLY). Indonesian also does not have verb 1, verb 2, and verb 3 like in English. (This makes it needs a lot of effort if a person whose mother tongue is Indonesian while learning English. He needs to choose which grammar of tense he needs to use in English, while in Indonesian he does not need to.)
To indicate or stress when an event happens, we usually use adverbs of time because a verb will be the same no matter when the action is done.
For example, you want to say "I am eating", you can say "saya SEDANG makan". The word "SEDANG" indicates the event is being done now. If you want to say "I have eaten", you say "saya SUDAH makan". The word "SUDAH" indicates the event has been done.
The word "makan" does not need to be changed into verb 2 or verb 3 or anything like that. You just need to add adverb(s) to the sentence(s) to avoid misunderstanding with the person you are talking to (about when the event happens).
So, you should add the word "sedang" to be able to indicate that you are eating, and it is being done (the moment you speak).
Hope this explanation helps! :))