What are some things i need to know about mating my guinea pig?

i have a hazel female guinea pig and my friend has a black and white male guinea pig. we want to mate our but im the one keeping the babies and some of my mom friends already called dibs on the ones born. my questions are at wat age can i mate her? how many babies will she have if this is her 1st birth? how long will she be preggo for? how will i know she is pregnant? and should i take her to a vet before she gets preggo, during she is preggo and after she has her babies to check how her and her kidz are doing? thanks for the answers :D