Well in your case it's very simple !
1. Install latest Android SDK from
HERE
2. Flash a custom RECOVERY.img from
HERE [press CTRL+F to find this : ClockworkMod Recovery 5.0.2.0 and click Download !]
3. Download a custom ROM ,you have many options ,i will give you one option ,it' the most used

> download
THIS and put it on your STORAGE [first line ,where it says Nexus S] ,then download
THIS [these are the Google apps that come separately;
4. Remember the custom recovery image downloaded ? [clockwork mod ...] ,rename that in : recovery.img [make sure if you have "view file extension" in Windows on or off so you won't rename it in something like recovery.img.img

! put it in your Android SDK platform-tools folder [like this : D:\Android\sdk\platform-tools ,and drop it there !];
5. MAKE A BACKUP OF YOUR APPS !!! Maybe use "titanium backup" > Market ?!
6. Put your Custom ROM on your storage !
7. Shut down phone ! > after shut down hold VOLUME UP key + POWER at the same time ! if at first you don't succeed ,try again

> You will enter the
Bootloader ! leave it like this !
8. You will see this on your phone :
BOOTLOADER > connect your phone to the PC ,after WIN installs your device GO TO : My Computer/Computer > Device Manager : you will see ADB composite device or Android "something" listed there > UNINSTALL IT ! [if you have WIN7 check the "delete driver" option !] now check for NEW COMPONENTS or reconnect the phone to the PC and update the device's driver from this here folder [might differ from yours ,check Android SDK path !!] : "D:\Android\sdk\extras\google\usb_driver" ,install it ,click on any ACCEPT/INSTALL ANYWAY button you might come across

) - it worked for me !
9. After installation click START ,go to RUN command and open CMD [just type cmd and press enter > now you will have to go to your "platform-tools" folder in CMD window

> follow these steps !
THIS IS AN EXAMPLE !! CHECK YOUR PATH TO "Android SDK" folder !!
path "D:\Android\sdk\platform-tools"
type in CMD : D: and ENTER;
type : cd Android\sdk\platform-tools and press enter / or if it does not work : cd android and enter , cd sdk and enter , cd platform-tools and enter;
type : fastboot devices [you should see some text under the line "list of devices attached" ,if YES ,your driver is OK ,if NOT ,ask for HELP]
type : fastboot flash recovery recovery.img enter > you will flash onto your device the custom recovery.img ! > you will see some text into the CMD window;
_____________________________________________________________
After all of this is done install your custom ROM :
-disconnect your phone from PC [
DO NOT RESTART/POWER OFF !!] ,using vol up/vol down select RECOVERY and press POWER to enter recovery mode !
IN RECOVERY MODE :
-using VOL UP/DOWN select wipe DATA and CACHE [2 options!] ,use POWER to select YES;
-go to ADVANCED and select wipe DALVIK and BATTERY;
-choose BACK and select INSTALL FROM SD CARD / CHOOSE FROM SD CARD and select your custom ROM that you put earlier on your storage !
-wait for it to install ,go BACK if you have to and select REBOOT
Congrats ! You have a rooted device !
I know there are lots of steps to follow ,but don't panic

you will get the hang of it after the first time.