Если существительное в единственном числе, то предложение начинается с It's, если в мн. числе - то с They're 

далее меняем 
кто? на чей?
I - my я - мой 
he - his он - его 
she - her она  -ее 
it - its  оно - ее 
we - our мы наш 
you - your вы ты - ваш твой 
they - their они - их

It's our family. 
They're your sisters. 
It's her grandfather. 
They're his cars. 
It's their garden. 
They're its toys. 
It's my computer. 
They're our friends.