Sorry, I am very new to electric guitars. I have a squier affinity strat. It has factory sss pickups. I want to achieve the sounds that Slash plays in Sweet Child o' mine. When I pluck two of the high strings at the same time on the higher frets, it sounds very muddy and distorted. Is it my guitar, amp, or do I need a pedal? My amp is a 10g fender frontman. I am thinking of getting humbuckers for it to see if it solves the problem.
It will sound muddy! Listen very, very carefully to the way Slash plays those notes. Nearly all of the time, he doesn't let one ring into the next. It's really more like speech; you can't say the next word till you've finished the previous one. The overlap, when there is some, is minimal, so the muddiness isn't allowed to happen.
This is always the problem with distortion/overdrive. The effect highlights harmonics, and often those of one note clash with those of another played simultaneously. Hence power chords work, whereas most other times when two or more notes are played it sounds muddy.
Just clean up your playing, be careful when one note finishes and the next starts, and it'll become great sounding, with whatever distortion you use. Other pedal effects may get you closer to the sound lusted after, but it's more in the execution of the playing.
As Tim already said, this has neither much to do with your guitar, amp, or (lack of) pedals but a lot with your playing. The phenomenon is technically called intermodulation distortion: whenever you play multiple different notes at the same time, the overdrive output contains not only the frequencies you put it, but also sums and differences of those frequencies. The difference frequencies are particularly problematic: when playing e.g. A4 on the 10th fret and E♭5 on the 11th, the frequencies are 440 Hz and 622 Hz. The difference of these is 182 Hz, which lies between F3 and F♯3 – more than an octave lower than the notes you're playing, thus it stands out very audible and unfortunately completely unharmonic to the rest.
So, when playing clear licks such as Sweet Child o' Mine on overdriven guitar, you must make sure that the strings don't ring into each other. Work on your muting technique!
Only in a few cases does intermodulation actually work out to good effect. The best known use are power chords: if you play the 165 Hz E3 together with the 247 Hz B3, the difference is 82 Hz, which is simply E2: exactly† an octave lower than the base note. That's why powerchords sound so nice and fat.
†Not quite. It would be exactly E2 in Pythagorean tuning, because the interval of a fifth is defined as ³⁄₂ there, hence you get ³⁄₂ – 1 = ½, which is an octave lower than the base note. On an instrument in 12-edo tuning (which is only an approximation to these just interval ratios), the fifths are very slightly flatter, but still ok for powerchords. All other just intervals are too crudely approximated by 12-edo to sound harmonic through heavy distortion, though this would not be the case if our instruments were set up for just intonation.