Creating functions to classify text can be as complex as you want.
A simple approach is to simply map each individual term to the index of the function. Then you present each document as a frequency vector of each term. (You can delete stop words, weight conditions, etc. Etc.). To classify the text, you also assign a label to each vector.
For example, if the document was a sentence:
John loves Mary
labeled spam.
Then you may have the following mapping:
John : 1
loves: 2
Mary: 3
Then your vector will look like this:
1 1 2 1 3 1
(I assumed that each trait has the weight of one)
SVM.NET, .