what is a soundcard? And what does the motherboard do and why should are more expensive motherboards Better
If you don't know what it is you won't need an aftermarket sound card. Makes no difference, pretty much a marketing gimmick.
Motherboards come with sound cards inside them.
The motherboard is where you plug all your parts in. the more expensive ones have more features but usually you are fine with a cheap one, they are much better quality than they used to be.
Also there's 3 sizes (pretty much...) mini ITX, micro ATX and ATX. that's from smallest to biggest. Depends on what size you want your pc. Most people have micro atx or atx.
The motherboard performs two tasks:
A cheap motherboard might have 2 RAM slots, 4 SATA ports, USB2.0 ports, a PCIe port and a PCI port whereas an expensive one might have 4 RAM slots, 10 SATA ports, USB 3.0 ports, two PCIe ports and 4 PCI ports.
The first would be fine for a budget build, but you would need the features of the second one for a build using multiple graphics cards, lots of RAM or a large number of drives.