Rule 1 - they go after the noun. Where we would say a black dog, in
Spanish this would be un perro negro (literally a dog black)
Rule 2 - the adjective must agree with the gender of the noun. Many adjectives end in -o when you find them in the dictionary. If you are talking about a feminine noun, then you change the ending to -a.
For example: un chico guapo una chica guapa
If you find an adjective in the dictionary that ends in a letter other than -o, (like importante), then it stays the same for both masculine and feminine nouns.
For example: un chico interesante una chica interesante
Rule 3 - the adjective must agree in number as well as gender. This means if you are talking about more than one noun, then you need to add an -S to the plural also.
For example: unos chicos interesantes unas chicas interesantes
unos chicos guapos unas chicas guapas