I rooted the Wildfire S from my girlfriend yesterday.
I am not a smartphone guy but i could manage.
Basic procedure is described in this forum.
You have to unlock the bootloader through a procedure from HTC
Then you have to flash a custom recovery software (clockworkmod)
This custom recovery software is accesible by the recovery menu entry in the bootloader.
Then this recovery SW will aquire root access by using a root.zip script.
You install Super user app from the market.
You install link2sd from the market.
You take out the SD card and put it into a card reader.
You put the card reader into your windows pc.
You take a partitioning program to make the existing fat32 partition smaller and create a new ext2 partition on the SD.
You put your SD card back into your phone.
Now link2sd can use the partition to link all user apps onto the SD (don't use normal app2sd now!!).
Now she has her major apps and updates installed and still 114 MB internal memory free with the original HTC ROMS.
This is just a very rough summary of the process to give you an overview. You have to check out the detailed description in this forum.
Check out this post for details:
Check out this post on how to set up the SD card for link2sd
PS: It took me some hours to follow all the instructions and read all the stuff.
PPS: You don't know what rooting is ? It is gaining administrator rights on your phone. Like the admin rights on your windows PC.