Generally the songs are played in the standard tuning especially in "E standard".
Below is the picture of a neck of a guitar showing the notes of different open strings. The part of neck between two metal ridges (actually called fret wires) as shown in the picture are called frets. Usually your guitar will have a marking at 3rd, 5th, 7th, 9th, 12th, 15th, 17th, 19th, 21st and 24th frets.
1st string (thinnest one - in the picture its tuned to E5)
6th string (thickest one- in the picture its tuned to E3)
Its easier to understand if I mention 6th string as just a bass version of the 1st string. They are the same notes with differences in resonating frequencies.
If you look at the second picture you discover that:
Pressing the 6th string in 5th fret and plucking the string is same as plucking the 5th open string (without any fingering). And similar is the case with other strings. But you need to remember that pressing the 3rd string in 5th fret and plucking is not similar to plucking the open 2nd string. So, this is how you tune your guitar. Start tuning from the 1st or the 6th string and go on playing the notes on the consecutive strings with the technique above. When your guitar is tuned and you pluck together: the 1st open string and 2nd string pressing on 5th fret then you mustn't hear two different sounds. If you have no idea about the note of the first string, just tune it to whatever you like and start tuning other strings based on that for practice. I have attached the sound of the 1st string played without fingering. You have to use your ear to first tune that string then the rest is really easy.
Now, just focus on the picture above and you can learn a lot from it.
Now tuning in "Drop D" isn't difficult at all. You just need to tune the 6th strings 1 step down i.e. two notes low. If you compare "standard E" and "drop D" tuning then:
Same thing applies for other drop tunings as well.
Strings Standard E Drop D
1st E E
2nd B B
3rd G G
4th D D
5th A A
6th E D