How can I Improve the quality of an amplifier?
I amplify using LM358 Op-amp and 2 resistors.


An LM358 is a really, really bad choice for driving an 8 ohm speaker. Typical output current is 40mA and into an 8 ohm load, the power delivered will be: -

\$I^2R = 0.04^2\times 8 = 12.8 mW\$

Also, you may have wired up your stereo inputs incorrectly and be cancelling out the mono part of the stereo signal - this tends to cancel most vocals, bass etc. i.e. anything that is centred in the stereo field.

Try using an LM386N-3 - it is capable of outputting 500 mW. You could do it with discrete transistors but I don't feel you are ready for this step just yet.