Setting Up New Smartphone...

I have been without a mobile phone. Change cometh.
I intend to purchase an unlocked device and have it running on GiffGaff Pay as you Go.
I need to get a new telephone number and set the whole thing up from the beginning.

Any help or links to help would be gratefully appreciated.
Many thanks,
s.

 

I just found this.
So I should be good to go.
I hope.
s.

 

Make sure the phone you purchase is compatible with the network providing service.

 

Thank you Just Playin, The phone I have in mind is new and unlocked. I asked the seller;

It it easy to place a pay as you go GiffGaff SIM card into this smartphone?
Apologies for the question but I'm new to Android Smartphones.
Thank you in advance,
s.
The answer came back:
Hi,
Yes you can put any network sim in this phone as the phone is unlocked.
Kind regards,

I have yet to buy the phone.
Thank you for your concern,
s.