What is the difference between:
heiße and heißt
das and die

Heiße is when you are talking about yourself, e.g. Ich heiße Rachael.
Heißt is when you are talking about someone else, e.g. Du heißt Ella.

Heiße and heißt are different conjugations of the word heißen - the end of the word changes depending on who the word is referring to. Here is the conjugation - 

ich heiße - I am called / My name is 
du heißt - You are called / Your name is 
er/sie/es heißt - He/she/it is called / his/her/its name is 
wir heißen - we are called / our name is 
ihr heißt - you (plural) are called / your (plural) name is 
sie heißen - they are called / their name is 
Sie heißen - you (formal) are called / your (formal) name is 

Das and die are different words that show the gender of the noun you are using. There is also a third word, der. 'The' becomes either der, die, or das, depending on the gender of the noun.

Der is masculine e.g. Der Hund 
Die is feminine e.g. Die Katze (die is also used to make something plural)
Das is neuter e.g. Das Hause. 

I hope this helps!