If you're plugging directly into the computer, then I'm not surprised that it sounds bad. The inside of a computer, desktop or laptop, is terrible for analog signals of any kind, including audio. Way too much digital noise running around in there to allow any sort of precision. That lack of precision appears as noise. You're literally listening to the electrical noise inside of your computer.
No point in designing to a better spec than a 1980's consumer tape recorder, so that's the actual performance of a built-in sound card. Even if it's one of those high-spec things marketed to audiophools, the noise that it has to deal with brings it right back down to an '80's cassette tape.
That's a big part of what a USB sound card is for - to get away from that noise - but the cheap ones of those are practically just a copy of what the computer has internally. Same cheap chip with cassette-tape performance, different plastic case.
Like I said, you want a USB thing that is (actually!) made for the pro market, not for consumers. And of course, use it like it's meant to be used. No funky adapters that destroy the critical parts of the design. (so you might need a new cord too)
For phantom power, most pro interfaces already include it, so make sure you get one of those, and don't worry about it any more except to turn that switch on.
The computer's built-in mic jack actually does have phantom power on it already, but it's a different format: 5V through 2kohms on a single pin, compared to 48V through 6.8kohms on two pins each. And depending on how the adapter cord is wired, you might have even more problems.
Get the gear to do it right, and do it right. I have a Behringer UMC202 that satisfies all of that, but it's not the only thing that does.
