You can either use regular chocolate chips, semi sweet or milk chocolate or you can buy the chocolate that is specifically for dipping, such as Dulci Frutta dipping chocolate that you can buy at any grocery store.If you use chocolate chips, melt either in the microwave or over a small pan of simmering water (you'll have more luck with dipping if you do it on the stove, melting in the microwave dries out the chocolate and cooks it too fast, especially if it's a big batch). Once melted I usually add a few tablespoons of butter, and mix until melted. It keeps it softer.
I usually have an assembly line of toppings ready, including, chopped pecans, chopped walnuts, chopped marshmallows, chocolate sprinkles, coconut, sugar sprinkles, and chopped chocolate chips. You can use anything you like, go crazy :)