I'm new to Linux and just installed Ubuntu.
I'm really enjoying it, so far.
But the ram usage is quite high, I think? The system is using 3.1GB RAM while I'm just browsing the internet.
Is this normal, and what can I do to lower it?
Ubuntu, its 'flavour' variations, and other Linux distros, will use as much RAM as available. It will also release that memory for other higher priority use as needed. This is normal.
Ubuntu uses as much of the available RAM as it needs in order to reduce wear on the hard drive(s) because the user's data is stored on the hard drive(s), and it is not always possible to restore all of the data that was stored on a faulty hard drive depending on whether or not that data was backed up. The objective is to protect the user's data. If a RAM stick wears out it can be replaced by another compatible RAM stick, and Ubuntu will continue to work properly without losing the user's data.
You can use Lubuntu desktop environment over normal Ubuntu. Once you install, you can choose the desktop environment while logging in. Lubuntu is a light weight desktop for Ubuntu and costs only 500 MB of RAM by default.
You can install it for free through Synaptic Package Manager ( which if not available can be found for free at the Ubuntu Store ) by simply searching for it.
You can also find a lot of 3rd party guides if you simply Google " Install lubuntu on 18.04 " or similar search terms.
There are also MATE, XFCE desktop environments which you can use in a similar manner. They consume much less resources than the default Gnome desktop. Gnome is interesting to use, you might find other DEs not that comfortable in the beginning. It is mainly because they lack those bells and whistles.
In the end, it is your choice. Do you need all those fun and decorations or get the work done. Good luck.
chromium
tab has many tens of tabs and thus accounts for the majority of my memory usage. A user on the Lubuntu discourse site (https://discourse.lubuntu.me/t/distro-compare-mem-use/740) recently posted a comparison of various distros, it provided some details whilst your quesiton provides none (I don't know your release, what desktop you are using, what you have set to autostart, what browser, how many tabs, what sites (fridge.ubuntu.com uses little, gmail.com uses heaps by example, etc) – guiverc Feb 22 '20 at 22:09