You can use Coke and Mentos but be careful because the second the two touch they will explode! It doesn't really flow out though, it really just sprays straight up. You can also use Baking Soda and Vinegar. To make it look more realistic you can dye the vinegar red. It is not as crazy and looks a lot more realistic.
First you make yo volcano than you need vinager and mix it with baking soda or pop and and put bubbles in it and make it half of the way of your volcano and it should it ex poled if not I am sorry but it took 5 time for me so never give up