These two verbs look similar and so can be easy to confuse! Let me break it down for you.
Set is verb that means to put or to place something somewhere (it has other meanings, but this is the only meaning we will focus on today). This verb is followed directly by and object and then a place. Another thing about set is that it is the same form in the past and in the present. For example:
A: Where is my book?
B: I don’t know. Earlier today, he set the book (the object) on the kitchen table (the place).
Set the groceries (the object) down over there on the counter (the place).
Sit is a verb that means to be seated (there are also more meanings, but for today, just this one). Unlike the verb set, it is never followed directly by and object without a preposition. It also changes forms from the present to the past: sit, sat, sat. For example:
Please, sit right here (no object, just a place).
The cat sat on (needs a preposition before the object) the mat.
Helpful tips: The verb set means that someone is picking something up, carrying it and putting it down. So, it isn’t often used for people because we usually sit without being picked up first! So, if you are doing the movement for something or someone because you are carrying them, then you would use set. If you are moving your own body, you should use sit. Just think: “I can sit by myself!”
I hope this was helpful :).