Well, I can open up and share things with people, but was almost completely unable to do so in my teens - so some of these things can be a bit age-related, and you can learn a lot..
You can first talk about other people (family or friends, or what you saw on TV/net/in books or magazines) or share stories from when you were a kid, or simple 'unthreatening' things.. like good memories or any funny/good things that happened.. then, you slowly get an opinion of that other person, when you say something and they say something.. It's important to start slowly and not with your 'darkest deepest secrets' even if that's all you've been thinking about.. There's a time and place for everything..
You need to know and trust people well before you trust them with the more vulnerable stuff.. (!!)
It's like dancing, a bit. You go one step ahead, then they go one step ahead.. or like playing ping-pong.. or running a marathon.. if you never ran one before, you don't go full 42 km in one day.. you start small, to build your level of fitness and ability to run..
small talk is the warm-up, if you find people are interested in talking more (body language, eye contact, smiling, questions, etc.) and you like them too, you can dance into the other 'arena' - favorite music bands or interests/hobbies.. this can get sorta personal, and people like talking about stuff they are interested in.. so you share a little, they share a little.. if you find good common interests, you can talk about those.. it's easier to express emotions in 'small doses' too.. and good emotions first, 'bad' emotions if it's really important too.. these are not so easy sometimes.. it's easiest if it's a shared experience: tired and exhausted cause studying for the exams = a common student interest, so many can relate..
You can also share ideas/opinions/emotions via discussing music or books or films/TV series or news.. 'Did you hear/see that new song/film/series?' or 'Did you hear about .....?' can show you what the other person thinks about a BIG range of topics.. (just apply news on what you want to find out their attitude about). Then, you know their opinion on something and you may know better whether to talk about certain things or not.
If you want to talk about something 'negative' ideally find someone also going through this or who has already gone through this and/or looks sympathetic. Then, you can both moan about it and it's easier
Or s/he will say, 'Ah, I've been through this too, and it can get easier' and maybe even share some tips
Or talk about wishes and plans, or any interesting developments or events in a field you're both interested in...