Hi JaceMindSculptor
God you're young! I feel like I'm giving advice to an embryo (joke)
There are lots of resources out there about improving conversational skills. Here are a few useful pointers
Learn to listen carefully. People appreciate others who are paying attention to them. Most people are not listening.
Learn to observe your surroundings and environment. Also learn to observe people.
Talk to anyone and everyone even if it's just to say Hello or Hi. One it will loosen you up and it also takes the pressure off any one conversation. I think this is especially useful when you are with a girl you like - you are social.
When talking with girls use open questions occasionally, this should get them to carry most of the conversation. Ask for opinions, ask for advice. Try to talk more about emotions or feelings than things or achievements.
Get yourself knowledgeable on current pop culture favourites , music, tv, films, games etc - this doesn't have to be in depth but enough so you are aware of whats in and out of fashion.
Start to enjoy reading and learning trivia about anything and everything.
Learn some good one liner or short jokes. Lot's of stuff online you can use.
Now 3 more 'advanced' tips
Try to match the pacing and style of any conversation - if it's fast you go fast, loud you go loud. Also, and this can be hard, try to keep the balance of conversation - if they are speaking for just 20 seconds do not reply by rambling on for 20 minutes!
Brainstorm conversation ideas. When you are alone just brainstorm topics. Think of one topic then write it down, think of anything you can associate with that topic and write those down and keep going. When you get stuck choose another topic and repeat. Check out resources on mind mapping
Learn how to steer conversations to topics you like and can be productive for you. Linked to the brainstorming as that sort of creates a map of topics you can use.
I would highly recommend you check out Carlos Xuma's - Alpha Male Conversation Skills product
http://www.alphaconfidence.com/conversation-persuasion/
http://www.alphaconfidence.com/conversation-persuasion/