Hi Lina! It's good to know about the things that you would like to sell in a garage sale. Selling your old clothes is a really good idea.

Here are some corrections:

I want to sell the clothes in a garage sale.
>>I want to sell my old clothes in a garage sale. (Use the if you already talked about which or whose clothes they are.)


Because I have a lot of clothes.
>>Correct!

But It's small for me.
>>But, they are small for me. (You have a lot of clothes so you need to use "they are")

I can give to my cousine, but It's big for her.
>>I can give them to my cousin but they are big for her. (Use them to talk about the item that you want to give to your cousin. And, use they are because you are talking about many clothes.)

So I think sell the clothes in a garage sale is good.
>>So, I think selling the clothes in a garage sale is good. (Use verb+ing if the verb functions as a noun in the sentence. It is called a Gerund.)