Hello @marlene54 & welcome to the giffgaff Community
The iPhone 5s takes a nano size SIM, if you current have one then you can go ahead and pop it in, if not you can Simply order another SIM from your spread page see here https://giffgaff.com/orders/mgm
All SIMs shipped by giffgaff are 3 in 1 (Standard , Micro and Nano) simply push out the middle form a nano size SIM for the iPhone 5s see this image http://s24.postimg.org/7t8vp0m5x/large.jpg
When the SIM arrives you will need to perform a SIM swap please see this guide http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084 on how to perform a SIM swap.
Carrying out the SIM swap will transfer your current number, airtime credit and/or bundle (goodybag,gigabag) over to new SIM.
The SIM swap should take no more than 30 minutes to complete and can be performed any time between 4.30am and 9.30pm.
Furthemore, all phones sold by giffgaff are 100% unlocked so any UK or international SIM card is accepted. If you wish to learn more about buying a phone from giffgaff please check out this knowledge base guide http://community.giffgaff.com/t5/Getting-started/Buying-a-giffgaff-phone/ta-p/10441209