Yes, a garment is one piece of clothing (a shirt, a pair of pants, etc.). You could also call a garment an "article of clothing."
"Outfit" refers to the clothes and accessories you wear at the same time. So yes, the t-shirt and pair of pants you described can be an outfit, if you wear them together. However, the word "outfit" can also describe clothes that match and are meant to be worn together - for instance, you might have a "work outfit" you often wear to work.
Clothing refers to the garments you wear - pants, shirts, sweaters, dresses, etc. There are other words you can use for other things you wear. For example, belts, hats, and scarves are "accessories" - they aren't garments, but you wear them over your clothing.
Instead of "a piece of pants," you would say "a pair of pants." Strange, yes, but like some other words in English ("a pair of eyeglasses," "a pair of scissors"), pants are referred to in the plural form because they made of two combined parts (two pant legs).
Under your pants, you wear "underwear."
"Pants" and "trousers" are the same thing.
A stuffed animal can be a dead animal that has been preserved, but we more often use "stuffed animal" for a soft toy animal.
Hope that helps!