1) We all friends have met up at one of our friend's place.(is this correct) We all met up at a friend's house OR A group of us met up at a friend's house.
2) I bought it two years ago/before (what to use) Which do you think? And why?
3) My exams are starting from/on tomorrow.(what to use) I would simply say "My exams are starting tomorrow".
4) All pictures are in /on her phone..(what to use) There may be a mixture of responses to this. Personally, I would say "All the pictures are on her phone."