You need to activate the SIM online using the 6 digit code on the card and then top-up £10. Activation can be within an hour but sometimes 24hrs. You will see your 'new' number on the mygiffgaff page (lhs) once activated or send the word number to 2020 after activated to see it.
Can't 'set' your own number but can transfer another one you have from another provider in (know as porting in).
You have to activate your sim,including topping up,online.Once
you've done this,activation should then take about 30 minutes
to fully complete but can take upto 24 hours-at which point you'll be allocated a random giffgaff number.This will appear under your usernam on your homepage.
If you want to transfer your current network provider's number to giffgaff,you'll have to contact them and ask for your PAC.Then fill in the transfer you number form found in the handy links section on your homepage.