You've got it backwards. A firewall blocks everything, then you open up the ports you want to use. A standard config would allow everything going out, and block everything coming in (unless you initiated that connection, then it is allowed).

So the question you should be asking, is what services do you think you're going to be running on your desktop that you plan to allow anyone on the internet to get to?

If you really need one take white list approach. Block everything you don't need and only open what you need. Have fun finding out what you need.

UFW

This is just my personal computer and I'm a newbie to configure firewalls

Leave it alone.

If you want to experiment, set up a VM and experiment there.

Also, if you want to learn about Linux firewalls, go for iptables instead. UFW is easier, yes, but you won't get the standard way of configuring a Linux firewall, though to be honest, unless you are directly connecting the computer to the internet, you probably won't need to bother.

And if you are working in an environment where you are dealing with a segmented network with limited access between segments, they will probably already use a separate firewall that is easier to manage centrally than induvidual firewalls running on individual computers

You shouldn't be touching it, honestly. There's a firewall at your router. It should be responsible for blocking incoming traffic. Firewalls on individual machines are for servers where you know exactly what's going in and out. I don't have a firewall on my desktop or laptop.

You will spend the best years of your life chasing random network connections if you block everything by default.

I think you might be looking for something like OpenSnitch.

You don't need a firewall on the LAN. It is just an annoyance to have to open ports later. Extra bureaucracy without benefits. This isn't Windows, you can can easily control your processes, choose if they bind to the network interface and on which port.