The uses of ser and estar are often a source of confusion, so let's review when each is used.

Uses of Ser:
to indicate profession. (Juan es un estudiante.)
to give the time or date. (Es la una y media.)
to denote ownership. (Este es el coche de Juan.)
to describe the material something is made of. (Los asientos son de cuero.)
to describe the origin of something. (Su coche es alemán.)
to give the location or time where something takes place. (La fiesta es hoy.)
when a noun follows the verb, use ser (El mejor coche es un descapotable.)
when an adjective follows the verb, and it describes the essence of the object. (El descapotable de Juan es rojo.)

Uses of Estar:
when the adjective follows the verb, and it describes the current condition of the object. (El descapotable está limpio.)
to give current location (El descapotable está en el taller mecánico.)

Here are some more examples of ser and estar in action.

Possession can be indicated in several ways in Spanish. The simplest way to denote possession is using de, in the same way one might use of to denote possession in English.
Los niños de Juan. (The children of Juan.)
El rey de España. (The king of Spain.)

Possession can also be denoted using el/la/los/las de, much as in English, possession may be denoted with that of or those of.
La niña es la de Juan. (The girl is that of Juan.)
Los llaves son las de Pedro. (The keys are thos of Pedro.)

Lastly, possession may be denoted with the possessive pronouns. The possessive pronouns in English are mine, yours, his, etc. Their equivalents in Spanish are el mio, el tuyo, el suyo, etc. All of the Spanish possessive pronouns are listed below. It is important to note that the possessive pronouns have several forms depending on gender and number. The choice of form is based on the gender and number of the object possessed, not the possessor. The form suyo can be ambiguous if the possessor is not obvious from the context of the rest of the sentance. If the possessor cannot be easily determined, the possessive pronoun should be omited, and possession should be denoted using de in one of the above forms.
El libro es el mio. (The book is mine.)
Juanita, los libros son los suyos. (Juanita, the books are yours.)
Pedro y Carlos, la llave es la suya. (Pedro and Carlos, the key is yours.)