See the tools in the Tool Shed category Assembly for available wrapped tools. These are for use in a local or cloud Galaxy. If you do not find the tool you want to use, it could be wrapped if you have programming support.
Tutorials that cover assembly can be found many places on the web. Using a tool in Galaxy will in most cases be very similar to using it line-command in terms of input requirements and parameters usage. There are also a few Galaxy-specific assembly training examples in the wiki at http://galaxyproject.org - see the section Learn. And this class series covers basic RNA-seq assembly in Galaxy, section 8: https://www.coursera.org/course/gengalaxy (not the same as DNA assembly, but could be a useful learning exercise anyway).
Hopefully this helps, Jen, Galaxy team