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.

Lots of good answers here but I’ll toss in my own “figure out what you need” experience from my first firewall funtime. (Disclaimer: I used nftables – it should be similar to ufw in terms of defaults though).

• Right off the bat, everything unneeded was blocked. I “needed” no configuration, except for maybe…
• Whatever CUPS runs on (when I use it)
• Sometimes I ran python -m http.server – I unblocked port 8000 for personal use.
• I chose to unblock port 53 (DNS). I wanted to connect to another computer via hostname IIRC (e.g. connecting to raspberry-pi.local. I might be misremembering this though).
• At one point I played with NGINX – that’s port 80 (HTTP) and port 443 (HTTPS).
• SSH was already permitted (port 22 – you need root access to enable traffic through ports below 1024 anyway so this wasn’t an issue for running typical apps)

I didn’t use WireShark back then, really. I think I just ran something like

sudo lsof -nP -iTCP -sTCP:LISTEN

which showed me a bunch of port traffic (mostly just harmless language servers).

You don’t have to dive to deep into all the “egress” and “ingress” and whatnot unless you’re doing something special. Or your software uses a weird port. (LocalSend lol)

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?

What can I do to see the intetnet traffic from individual apps

Wireshark

what I might want to block?

One strategy is to block everything, and open ports as needed. Beware that most guides focus on inbound traffic, whereas you seem to be focusing on outbound traffic.

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.

You don’t need a firewall on a typical desktop computer. You only need them on routers and servers.

That is because your personal computer is not actually on the internet. It is on a local network (LAN) and it talks only to your router. The router is the computer connected to the internet, and it has a firewall.

The question highlights a classic misunderstanding about networking that IMO should be better addressed. I was like OP once, and panicking about this pointlessly.

Addendum: You’re all replying to OP as if they’re a sysadmin managing a public-facing server. But OP says clearly that they’re just a beginner on a PC - which will almost certainly be firewalled by their router. We should be encouraging and educating people like this, not terrorizing them about all the risks they’re taking.