An acoustic guitar has 6 strings, hence the “6 string” 🙂
It starts from the E string (this is the string with the lowest pitch, or lowest frequency note), then the A string, followed by the D, G, B strings. And the last one is the e string. This one is the string with the highest pitch. This e sting produces the “E” note of the next octave begining from the base E string.

We’ll denote it as.., This is the plan view (top view) of the guitar, facing the strings.

e +---------------+
B +---------------+
G +---------------+
D +---------------+
A +---------------+
E +---------------+

Now hold the guitar as you generally hold it, the body of it on the lap, and your base of your right hand (considering your are right-handed) on the bridge. Bridge is the point from where the guitar strings start, it is NOT the end where you tune the string with the knobs.

Hold your thumb and index finger as you hold a pen, withing that grip hold the plectrum (pick). The blade of the pick should be at an angle to the strings, and not directly flat to the strings.

Now practice picking the strings, begining from the base E string to the high e string. All the picking should be down strokes. Go in an easy manner, till you get a feel of distances between the strings, etc…

Once you are comfortable with the simple picking start picking alternate strings. i.e. pick E, D, then A, G, then D, B, and G, e. and then back up again. This will help you in getting a hold of picking alternate, different strings.

I’ll explain you a notation that you will follow all over, It is not my notation, probably you must have seen it somewhere.
When I say you play C note, I’ll specify a fret. A fret is marking on which you play a note, to play a note you press your finger just before the nth fret (the fret that is mentioned to produce the note).
So…

e +---------------+
B +---------------+
G +---------------+
D +----0----------+
A +--3------------+
E +---------------+

..means play the 3rd fret on the A string to produce the C note, then the play open D string (i.e do not fret the string anywhere) to play the D note.