A fixed IP is different to a dynamic IP in the sense that the your service provider assigns the same IP address all the time when you get connected to the internet while dynamic IP is different every time you get connected.
Choosing your IP address
Only the FAI who own the IP adresses can choose for you.
Choosing between IP Fixed IP and dynamic?
Dynamic IP address is to your advantage in many ways:
First of all, having a fixed IP address ease hackers to detect you as your machine
is constantly at a specific IP address compared to dynamic IP which is changed constantly.
Having a fixed IP can make attempts to your private life in the sense even if you change your pseudonyms for a specific reason, your IP address will be refer to the same which will be easily detected.
Nowadays it is not necessary to know its IP address for connecting to your computer from the internet. You can use a dynamic DNS service (dyndng.org, no-ip.com, etc.), by associate a name to your fixed IP address for each connection. These services are usually free.
As done usually, instead of connecting on an address such as 18.104.22.168, you connect to an address type mylogin.dyndns.org. You will no more have to memorize in advance the IP address!
Obtaining a fixed IP address
A fixed IP address can be provided to you by your internet provider. Your internet provider will be the one who will decide upon providing a fixed IP address.
You should however consider that for many internet providers, it is quite impossible to opt for dynamic IP once you have changed for a fixed IP.