Hi there,

i am a beginner and i recently changed my strings. i had some trouble but i changed them.. i have a strat type guitar... Yamaha erg121C to be exact. I got the strings with the guitar.. they were packaged together.. so i guess they are the correct one.. so when i play like A string with D string... the A string buzzes but only when i am pressing the A string.. i went upto fret 5 and then it went away.. is there a reason y? also the E string buzzes when on a lower Tuning like D drop C.

Your guitar isn't set up right. Learn to set it up, there's a really good article on it somewhere here on UG. It's also a good opportunity to check the intonation on your guitar as well. Though if I had to guess, I'd say you need to raise your bridge. Also, it appears your guitar has some sort of trem system which means down-tuning would probably require a complete re-setup for it to play properly.

It might also have something to do with neck relief though if you're not sure what you're doing, don't mess with the truss rod.

Long story short, your guitar probably needs a set up so either bring it to a guitar technician/shop, get a friend who knows how to set up a guitar or read up on it yourself. Not too difficult once you know what to watch out for.