Please, first of all, notice that writing a virus is not a simple task for beginners. You must learn programming language and understand simple programming algorithms. You're pretty young, so I suppose your head can't take those all. You'll need a lot of practice.
Step 1. get a website. Step 2. throw coding at a wall until you find something that can crash a computer. Step 3. Put the virus into a program. Step 4. Disguise your website. (Good ideas are: Weight loss tips, Browser toys, and Video downloaders.) Step 5. Put the virus into a large button on the website. Step 6. Congratulations!!! You are the worst!!!