While your username is fixed after you have signed up and changing it is difficult, you can set an alternative nickname in your user preferences. This name is used for your signature, which you can enter by typing ~~~, or ~~~~ to get an additional timestamp. So if your username is "Klaus Meier", you could sign with "Klausi" by entering this into the nickname field. The software adds [[User:Name| to the beginning of the nickname, and ]] to the end, so this would become [[User:Klaus Meier|Klausi]]. By entering something like Klaus Meier]] [[User talk:Klaus Meier|(talk), you can "trick" the software into adding another link at the end, in this case, to the talk page, which will look like this: Klaus Meier (talk).
Read more: Setting your user preferences