Easy. Just sit in your chair at your PC, drink lots of liquid and relax...
Eventually, you'll need to pee, but dont goto the bathroom. Just keep on drinking...
When you feel you REALLY need to get up...
DONT..
JUST RELAX...
It will be hard at first, cause its unnatural - Thats understandable - However, with practise, it gets easier...
Eventually, you cant hold out any longer, so relax and let go. Focus your mind, upon how your dry clothing and chair are now getting wet and warm with your hot piss... Enjoy and indulge your mind in your bad behaviour...
Feel your warm piss forming a puddle under your arse, and squish around, getting your balls and arse-hole covered in nice fresh piss.. Keep going...
Before long, it will overflow your chair, and with luck, follow your jeans into your shoes and socks... This is a
GOOD BIT... Enjoy the warmth as it fills your shoes, and soaks your socks...
Chances are, you'll be finished at this point, but if you can
REALLY relax, and let a duker go, then do it... After all, you have just pissed yourself for fun and pleasure, and so why not go the full monty, and shit yourself as well?
ENJOY !!!
PS: This WONT help you with your problem, but you DID say what should you do next,.. and the above IS my suggestion ! 
ENJOY!