Yes. You should tell someone. Because being alone sucks.
If you've been struggling for enough time that you're googling this, then you probably know somewhere in your heart that you should reach out and talk to someone.
Here are some things that might help
1) Start with a friend first. Start with someone you feel is safe or someone you know has your back.
2) Think about what you need from this person. Do you need someone who is going to tell it to you straight or do you need someone who will listen and just be there? Maybe it's someone you can cry with? Try not to go to someone you know is not likely to give you the thing you need - it's probably a recipe for a bad conversation.
3) If you're a kid, you should figure out when and how to tell your parents. If this is scary, come prepared. Take an online mental health screen and bring them the results. Tell them you want to feel better. Ask them if mental illnesses run in the family. Expect that they might feel guilty and might not respond the way you want them to (your parents are human too).
4) If you don't have someone you think you can tell - you can still tell a stranger - someone in a forum like Inspire. This will help break the ice in your mind and maybe help you clarify things in your mind and get you started on your next step.
Telling someone is the first step to feeling better.