Oh Santa I'm so sorry to hear your sad news. I know how you feel as I lost my mum after a long illness when I was 17.
I found the best way of coping was to keep really busy for the first few days until the shock has died down ( I went straight back to school the next day), even though your mum had been ill for a while it is still a shock to the system.
If you keep busy over the next few days, and get all the official stuff and post mortem out of the way, you can then sit back and go through the grieving process by remembering the happy times you had with your mum when the atmosphere is much more relaxed.