There are a few ways you can make your own volcano - you can even make them erupt! The easiest way is to use a cone of paper or card - here's a template that has a cutaway of inside the volcano.
The classic choice is to make it with papier macher;
See this think for more info;
You can also use card to make an erupting volcano;
Or even sand, clay or play doh!!
If you really want to challenge yourself, you can make a complete replica of Mount Fuji in Japan that can be split in half by following this template here - warning, you'll have to be really good at making models!
You could even make a Lego volcano!
Or even Minecraft!