The preposition "con" 
"Con", has the basic meaning of "with", most of the times it is the equivalent of the English "with"
"with" is  its usual function but  it is somewhat more versatile than the English "with," 
so, you will see "con"  used in many cases where you probably wouldn't use "with" in English. 
This preposition  combines  with the personal pronouns and  alters its form when it contracts with them.
The personal pronoms  are: mí, ti, sí
they contract with "con"  in the following manner,  and become : conmigo, contigo and consigo, respectively . 
Note that  :
con + mí ( with me ) becomes "conmigo" (not : con mí) Ven conmigo Come with mí
con + tí ( with you ) becomes "contigo" (not: con ti) No puedo ir contigo I can't go with you
con + sí ( with him,her becomes " consigo" (not : con sí) El guarda el secreto consigo He keeps the secret with him.
you, himself, etc,)
Hoy los niños vienen conmigo. Today the children are coming with me.
El fin de semana lo pasarán contigo. They will spend the week-end with you.
Ven conmigo ahora, puedes hacer eso después. Come with me now, you can do that later.
Alejandra debe limpiar contigo este salón  Alejandra should clean with you this showroom.
Mario no quiere estudiar contigo.   Mario doesn't want to study with you. 
César desea bailar conmigo.  César wants to dance with me.
Fernando lleva las herramientas consigo. Fernando is taking the tools with him.
The preposition  is used with certain verbs where a different preposition (or none) is used in English. 
cases where "con"  is used in a way that differs from the use of "with" in English. 
Es necesario terminar con el proyecto. It is necessary to put an end to the proyect.
El auto chocó con el camión.  The car crashed into the truck.
Tengo que enfrentarme con el problema.  I have to face up to the problem.
Muchas veces sueño con el mar.  I often dream about the sea.
Quiero encontrarme con mis primas.  I want to run into my cousins.
Festejamos con unos amigos. We  celebrate with friends.
Las niñas juegan con las muñecas The girls plays with the dolls.
Puedo contar con mis amigos.  I can count on my friends.
Puedes contar conmigo. You can count on me.
Quiero casarme contigo. I want to get married with you.
Comimos un bife con ensalada. We ate a steak with salad.
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