When someone has an inherently negative outlook on life, it's difficult to change this, but it can be done. I often find that the problem stems from the fact that I'm not happy (well back when I had this sort of problem). I was always pessimistic because I had other problems that affected me. I didn't really do much with my friends apart from hang out in school and my weekends were relatively boring. Now seeing as how you didn't give much information I'll just try and explain a few instances.
Friends: Back then I was quite close to my friends, but there were only a select few of them, and even then we just talked in school and stuff but never really did anything much socially on a regular basis. It helped me a lot when all the idiots and brats left at the end of Year 11 (no compulsory education after that). I eventually became friends with lots of other people as the classes were know totally new and I talked to people that I didn't usually. Here you can improve too. Try to mix with other groups of people if at all possible, or maybe approach a person in your class that you wouldn't normally to ask for help on a question (just blag it). This is the easiest way to start to befriend someone and as you become friends you'll have a variety of other things to talk about.
I find friendship to be a huge importance in how happy I am. Just joking with them and talking to them about stuff can seriously make a boring and sad day so much better. It helped me get through the day and my mood gradually increased over time. If you're the humourous sort of person, make jokes in class or with friends to try and put a smile on yours and others' faces.
Social life: While I on occasion did go out and do stuff, my weekends were relatively mundane and I spent a lot of time alone in my room or doing other stuff by myself. I think it really helps that if you go out with your friends more often, or meet up. Even get some friends round your house or go out for a movie, bowling, etc. This allows you to relax and enjoy yourself. Barbecues are also a great social activity. It's so much fun to go to a park or a friends house and set up a barbecue just sitting around chilling to music, talking and playing games (and eating). The more friends you invite the funner it turns out to be. There's lots more things you can do with more people and lots of stuff you can talk about.
Now whilst I certainly don't encourage drinking underage, I found that attending parties also opened a new door to me. Not only was I able to have fun and dance to the music, but I got to get chill out and have fun with them. Now obviously if you don't drink then this won't be of much use, because it is no fun just standing around and watching drunk people dance, however you may still benefit from it if you are the dancing type of person. There will most likely be other non-drinking people there, and you can always stir up a conversation with them, or just hit the dancefloor.
Family life: How well do you get on with your family? I think that whilst some people hate their families and spend a lot of time in their rooms, it is important to talk to your family and do family activities, however infrequently you do them. You see your family every day and some people just say a few words to them. One night suggest you all watch a lighthearted comedy with each other, or perhaps play some type of board game. Even talking to each other about their day during dinner time allows you to engage in conversation and just get thoughts flowing.
I know these are fairly broad points and may not specifically address the topic, but I think there is no simple way to just adjust one's outlook on life. I think a person's pessimism or optimism is more to do with how happy they are. More often than not the more happy I feel the more positive thoughts I have about things and life in general.
I hope this helps, feel free to PM me anytime if you want to talk.
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and victorious in war shall be made glorious in peace.