software design process selection?

[Stephen Ting]

Recently I read about Extreme programming and find that it's quite
interesting and emphasize very much on testing + simplicity. Besides,
there are others planned design process likes UML. I am new to both the
design processes. 
 
Can anyone give any suggestions + guidelines on BEST PRACTICE in doing
software design and analysis using above mentioned methodologies or
other ways that is more efficient?  
 
Pls help!!!
 
Thanks
 
Stephen