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Years ago I heard this tutor comment, "If you play an accoustic, you're stuck with the tone of that accoustic." This is only part true.
For the moment, we'll stick to the steel-stringed accoustic. The factors involved are: the instrument, strings, plectrum.
The Flattop usually has a soundhole. The 'Dreadnought' or 'Jumbo' pattern has a big, warm sound with a powerful bass. If you hammer out big, basic chords to accompany voice, it's fine. You can get sweet riffs, but also "mud"-a cacophony of voicings (chord tones) that can collide with bass, lead guitar, and vocals. The warmth of the sound can sink in background noise etc in pub, club, or street.
The Archtop has 'F' holes top and bottom. The body is thinner, bridge and fretboard bearing a radius (some flattops have these features). The tone is "brilliant"-sharp and cutting-good for single-note, jazzy runs.
Strings The gauge is all we'll deal with here. Light = bending; Heavy = power. Somewhere between will be your gauge. A change may affect tuning. Light strings may go flat. Check that the natural harmonic at the 12th fret = the fretted note. If the fretted note is flat, the distance between the 12th fret and the saddle on the bridge has to be shortened. This can be done with a filed piece of ivory (artificial by law) jammed in front of the saddle. If the note is sharp, the ivory goes behind to lengthen the string section. You can bodge it with hard wood, but if you're on stage, don't (unless you're in a grunge outfit).
The Action Controlled by the truss rod. Quarter turns at a time. A Low action = speed and playability; a High action = volume. Test carefully for buzzing frets and/or 'convex' fingerboard (too tight). Beware.
Plectrum Try all that's available. I find nylon gives me the best range. Tortex gives more attack, shell more still. And as for shape--
Whoa--'range', 'attack'--what do these terms mean? Well, go to a good shop and you can pick your own. Gimmicks apart, ordinary ones are cheap. Only you know what you're looking for. When you've found it/them, get spares. If you're on the stand, tape spares to the back of your guitar in case you drop one. Got one? Got it in your hand?
How do I hold it? The standard way is: form a loose fist, hold pick between thumb and the bent 1st joint of your forefinger. Play with 'free' (unsupported) wrist. Mmm. Difficult? Yes. Django probably did this; many don't. Control comes with practice.
Try this .
Hold between thumb and forefinger. Let your fourth finger brush the finger plate below the strings; at first, you'll find your fourth finger will be 'superglued' to the top in rigid support of your hand. Not good. Too restricting. It's like riding a two-wheeled bike; gradually, you'll rely on support less and less, until you convince yourself you can 'ride', even 'fly', without falling off.
For Tone Variation, read the next page...