Here is some info about the P0305 Diagnostic Trouble code: http://repairpal.com/OBD-Code-P0305
You'll see that there are several things that can cause this code. If you are going to replace the spark plug, you might as well replace them all.
To get directions, you can look here for a free one: http://bit.ly/autozone_repair_info or go to http://bit.ly/alldata_repair_manuals or http://bit.ly/mitchell_repair_manuals_online and get an online repair manual subscription.
Just beware, some repair procedures are left out, because there is an assumption of minimal experience, and this may apply.