|>>|| No. 24771
>it all seems quite complicated
That's because it is. Audio recording is an art in itself, if you want professional results then you need to invest the time to learn. Without a reasonable understanding of the theory, you won't be able to get consistently good results.
>What does a USB interface actually do to the audio signals, though?
The same thing as your soundcard, only at higher quality and with the electronics needed to operate with proper microphones. Studio microphones are very different to the electret capsules found on gaming headsets and require careful preamplification. The type most generally suitable for vocal recording (condenser microphones) need 48v phantom power, which isn't supplied by consumer sound cards. The better electronics in a dedicated audio interface provide higher dynamic range and lower noise.
If you only need to record one thing at a time, there's a simple and relatively inexpensive option - a USB microphone. These are studio-quality microphones which have a preamp and audio interface built in.
There are a variety of these mics with a wide range of prices. At the lower end, I'd suggest the Samson Meteorite (£20) or the Samson CO1U (£55). If you're looking for bona-fide studio quality, then you'll need to spend a bit more - about £120 for an Audio Technica AT2020USB, a Rode NT-USB or an sE X1.
As >>24766 says, once you've got a quality mic then you need to sort out your room acoustics. Hanging a duvet behind your head will get you half way there. A Reflexion Filter (£50 - £120) will do wonders if you have the budget for one, or use another duvet behind the mic. A pop filter is essential with a condenser mic, but you can make one yourself with a wire coat hanger and an old pair of tights. You'll want to locate your computer as far away from the mic as possible to cut down on fan and hard drive noise.
If you want to learn more, consult the archives of Sound on Sound magazine. It's the definitive publication on home recording and their articles cover pretty much everything you need to know about recording, editing and mixing. If you'd prefer to have the information laid out for you in a course format, then I'd suggest the books Recording Secrets for the Small Studio and Mixing Secrets for the Small Studio by Mike Senior, which are the best introductory texts available.