I often having a lot of trouble describing situations. For example, I often say something but I actually meant something else and I don't really realise it till someone tells me. I'm 15 Years old, Is there something wrong with me??
I doubt it. You're young and going through puberty and sometimes that causes one to be embarrassed and get tongue tied. If you read the questions on here you should notice a fair number of people who can't ask a question if their life depended on it. So much so that we don't know what they are talking about and they provide no information at all.
You can go in your room and turn on some music and talk softly. Just describe 2 or 3 scenes you observed that day. Speaking slowly, don't get bogged down in details, and remember you are describing something that is unknown to the listener so don't expect them to understand where the action took place or who was involved.
Just practice every day. You'll get better.
Good luck and have a very descriptive 2018!