Fractal could be a lot... there are a lot of examples and codes out there, just search for it or start with the examples about "fractal and l-systems" here : http://processing.org/learning/topics/

after you are familiar with it, maybe think about ways to form letters... i guess it depends on how you want it to look like, what system you use and so one...

for example. you can use an black and white image as source, or a libarie like http://www.ricardmarxer.com/processing/geomerative/documentation/ to get the outlines of a text font. you can then use these outlines or letters to start your fractal system there, or maybe start it withing the letter and make it only grow within the outline... i dont know, i guess there are several ways to start. maybe come up with an idea&/design first and then it is easier to tell you what a good way would be...