"This" is for describing a single item or items that cannot be counted (like sand) in your possession. "These" is for multiple items also in your possession. "That" is for a single item that you do not possess. "Those" is for multiple items that you do not possess. This stick. These marbles. That car. Also used like This is. These are. That is. Those are. Hopefully that makes sense.

this = one object here these = many objects here that = one object there those = many objects there

This use to near singular That use to far singular These use to near plural Those use to far plural

Great question, Raiane! Note that the difference in pronunciation between "this" and "these" is obvious to a native English speaker, but very subtle to speakers of many other languages. At the end of this comment, I'm including a link to the relevant entry in the Oxford dictionary, where you can see the IPA phonetic transcription, as well as buttons you can click on to hear the pronunciations. http://oald8.oxfordlearnersdictionaries.com/dictionary/this