If you can identify the specific model of your graphics card and motherboard as accurately as possible that'll help. I'd guess 900 series has a wide variation in power use and performance.

In linux you'd be able to go into shell and run
lspci |grep -i vga
or
lspci |grep -i nvidia

In this case lspci is a command to " list pci devices"
On it's own that might give you a long list of sound, usb, network etc devices, so we "pipe" the output using '|' into the grep command. grep will filter by text pattern.
the '-i' makes it case insensitive search and 'vga' or 'invidia' is the sequence of characters to filter by.

Alternatively most desktop environments will have a gui option like "info centre" in kde.
Windows will probably have something too if you only have that, unless the'yre replaced it with coprolite trying to guess the answer.

For motherboard and PSU (power supply unit) you might need to open it up and look for labels or text on manufacturer and model.
For the motherboard, you really just need to know what type of pci it has but getting the manual online will be best.

For the PSU you need to know power rating in Watts and maybe "efficiency rating" usually as some metallic rating like gold, silver, bronze. The manufacturer helps determine how reliable the ratings are.

If you go inside you may want a torch, a compressed air can, a dust mask, to be outside. and/or a gentle vacuum cleaner (with a mesh or filter or tissue over the end just in case). or duster or toothbrush or something. If you've a nice clean environment that may all be no big deal, but if you have household dust, there can be a lot of build up in there over 12 years. Best not to breathe too much of that in.

To learn more there are loads of webshites and videos. I'd search for terms like "diy build pc ".
try to focus on low power or low to mid price to avoid lots of crap top end bullshit.
Look at some from back in the day. they should go step by step through components, and how they are installed and connected.

I'd say try explainingcomputers.com - this is both webpage and links to youtube.