I dont have any money for a new decent amp, So I thought that maybe should I buy an audio interface and use it in pc with Guitar Rig..

meanwhile, I connect my guitar to mic input through my pc sounds card, and I get a lot of buzzing and crappy sound, if I'll connect guitar through audio interface will I have better sound?
or should I need a new amp? Im asking because I saw alot of covers with audio interface and people are also connecting it to their amp and then audio interface..
Yes an audio interface would greatly help.
Go into the recording section and read the "introduction to recording" sticky.

What you've seen is guitarists plugging their guitars into their amps, then place a mic near the amp and plug the mic into the interface.
Using an amp sim is a much better option if you're looking for convenience.
Name's Luca.

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