It's used before a masculine noun starting with a vowel sound or a silent "h".
Here are some examples:
* Cet arbre est grand. (This tree is big.)
* Cet homme est gentil. (This man is kind.)
* Cet hôtel est magnifique. (This hotel is magnificent.)
Note: When using "cet" before a masculine noun starting with a consonant sound, you use "ce" instead.
* Ce livre est intéressant. (This book is interesting.)
* Ce chien est adorable. (This dog is adorable.)