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PHP Code knowledge is a plus.
Knowledge about Object Oriented programming is a big plus, too.
Apart from that, I would recommend that you
a) Have a very good idea about what you want to accomplish witht the module
b) Have a slight idea as to how the pages can be displayed
c) Pick a module to look after - I recommend the News module. It has taught me a great deal in how to work with XOOPS
d) Ask here with help for specific problems - but do try to take a stab at it first