Plectrums come in different sizes and thicknesses. Tortoise shell was the traditional material but most are made of plastic nowadays. I know one great jazz player in Australia who uses wooden picks, apparently like the famous Django Reinhardt used to play with.
One of the best ways to practice strumming and keeping time, is to play along to records of music you like. Try to get into the habit early though of knowing what you're playing — try to put a name to that chord you've just learned by ear. It will make it easier to fit the pieces of the puzzle together later on when you've got a collection. It's easier than you think. Listen for the bass note. It's usually the root of the chord, so if it's G, you can be pretty sure the chord will be some sort of a G chord. Test out the various flavors, like major, minor, seventh until it sounds right.
Strumming is the act of brushing across the strings of the guitar in order to make them ring out. This can be done with either a plectrum (pick) or with the fingers. There are countless ways of doing it, of course, so don't feel that there is a right or wrong way to strum. The only aim is to make music and we all come up with our own way of doing that, but they all require you to brush over the strings.
When you hear the term 'rhythm guitar', it usually refers to strumming because the various strumming patterns create rhythms. And just what are the musical entities that we strum? Chords. Chords are the main structure of any piece of music, and the job of a rhythm guitarist is to lay down a bed of chords for the other players — or singer — to work over. They're the 'background color'.
There is nothing mysterious about strumming. Whether you use your fingers or a flat pick, your hand must come down across the strings (down stroke), then it must come back up to perform another down stroke. You can let your fingers/pick brush across the strings on the way back up (up stroke). The combinations of both down strokes and up strokes are what strumming is all about. Time can be divided and subdivided into many slices, so you can see that between upstrokes and down strokes, and the number of divisions in the time-line, there are an infinity of different combinations. Most players find, however, that just a few well practiced patterns will get you through just about any tune, and that in fact the job of the rhythm player is to keep things clean, steady and uncluttered. The old 'KISS' principle —Keep it Simple, Stupid—can certainly apply when it comes to playing rhythm guitar. Watch the movies above to get started.
It may take you a while to find the most comfortable way of holding a flat pick. They do tend to rotate in your grip and of course they're easy to drop. More often than not, they wind up inside your guitar, so when you buy them, buy a few. I like the thinner, more flexible, picks, and a light touch is needed if you use them. Experiment with the angle at which the pick meets the strings. To minimize any scraping noise, you really want the tip to make contact, not the side or edge. Again, there are no hard-fast rules about this; over time, you will find your own, most comfortable way of holding the pick.
The harder you play, the heavier the pick. Players who specialize in rhythm guitar tend to strum hard, use heavier strings and a thicker, sturdier pick. You get much more volume and less rattling by doing so. If you want to combine a bit of both styles—strumming chords and playing single note lines—you'll probably find lighter strings and thinner picks are better. The one thing that applies to any style, though, is relaxed wrist and forearm. Your pick needs to dance over those strings, the upstrokes and down strokes need to flow. Practice, as always, is the key. New muscles will come into play and they to be taught how to behave. After enough practice, those muscles begin to remember on their own, without input from the brain. That's when we start playing, not working, the guitar.
4/4 time is by far the most common time signature. Rock, Blues, Pop, Funk, Reggae, R&B and Jazz pieces are usually written in 4/4 time. 3/4 is more the domain of ballads, country, folk and classical music. That's not a rule, by any means, but generally speaking, that's the way it breaks down. 3/4 time will always have a 'lilt' about it, something you may not want to hear in a Death Metal or Heavy Rock tune. The movies will help you to see how Time gets divided up for these two most common time signatures, but strumming is very much a 'feel' thing. Don't get too hung up on the exact breakdowns of patterns. Once you get the basics down, explore the possibilities just by playing along to your favorite tunes. Imitate what you're hearing, experiment with it all.
Palm muting is a technique where you lay the heel of you strumming hand over the strings near the bridge in order to dampen the ringing. This is done when you want a thumpy, percussive sound rather than a ringing, clear sound. The more pressure you apply, the less you will hear the note, so you'll need to experiment.
The distance away from the bridge that you lay your hand also affects the sound dramatically, so experiment with that too. By combining muted beats with open, ringing beats, you can come up with all manner of interesting and effective textures. The movie above shows a few ways of using this common technique.
You don't need a plectrum to strum. If you get into playing finger stlye, you don't want to have to reach for a pick every time you want to strum through a passage, can just use your thumb and finger tips. Once again, there are many ways to do this: you can just use the thumb, doing up strokes and down strokes.
you can use the thumb for down strokes and the fingertip (usually the index finger) for the up strokes; you can use the thumb and the back of the index fingers for downstrokes (sometimes referred to as 'frailing') with the fingertips doing the up strokes ... or any combination of those. The movie shows some of the ways I know of doing it.