Like just about anything, strumming (and coming up with strumming patterns) can be learned fairly easily. But, and again like just about anything, how good you get will depend on how much effort you put into it. Some people are naturally talented in this area while others will have to work at it.
Surprisingly, the quickest way to learn how to strum doesn't even involve touching your guitar. Really. Sit back and listen to some music. Any music will do, but if it just so happens to be a song you're trying to learn, then more power to you. Now, listen. Hear the beat. Listen to the drums and the bass. Tap your feet with the rhythm. Take whichever hand you strum with, rest it on your thigh and tap out a pattern. Start simply … really simply if you have to. Even if it's just tapping out every beat or every other beat. Once you have a pattern you like, repeat it until you are happy with your consistency. Are you able to keep up with the other instruments? Does your pattern copy theirs? If not (and it really doesn't have to), does it add to the overall rhythm? Does it overwhelm the rhythm?
Here's a trade secret for strumming. Get a snare drum book. It will teach just about every possible 4/4 pattern there is. Just pick a bar chord and follow the pattern of note. Use a rake and muffle the strings for the rests. Try shifting to different chords during these patterns. Go through the whole book and that should cover just about every rhythm out there. Do each exercise until you don't think about it anymore. It should go from a mathematical counting pattern to a feel. Once you feel the rhythm, move on to the next exercise.
As a rule, the more people you've got playing, the less space there is to play around with. And the more people you have playing the same instruments (guitars more than likely) the easier it is for things to get muddy really quickly. The ability to hear and come up with appropriate rhythm parts cannot be passed over lightly.
And that's just in playing for the fun of it. If your aspirations involve professional work, whether solo or with a band, then your ability to keep appropriate, interesting and varied rhythms are a must. I should point out here that it's not only beginners who have trouble figuring out a good strumming pattern. But, bless their hearts, they are the only ones who seem to be willing to admit it.
We can tackle this difficult problem from many different angles, but let's go after the easy solution first. You not only have to be able to keep time, but you also have to appreciate why you keep time. Time is what ultimately holds a song together. I can't tell you how often I've been out to see musicians or bands and have noticed that there seems to be a huge perception gap between a “great,” “good” or “okay” act. Sometimes the “good” and “okay” bands actually have better musical talent than the “great” bands but they just don't come across all that well. More likely than not, it is simply a matter of the group being in sync with each other. Simple things like starting and ending at the same places or giving dynamic changes in tone, volume or rhythm together as a unit have a powerfully positive effect on the audience. The average listener probably won't be able to tell you that the lead player muffed a note or that the rhythm guitarist played the wrong chord for a brief moment (unless it was really wrong and really loud). But he or she will be able to tell you if the drums, bass and guitar are all playing at different tempos.
Regardless of your level of ability, you should own a metronome, which is a device that beats time at a pace of your choosing. Like everything else these days, there is no end to the various types you can get. I wouldn't be surprised to find you could download one from the net. Let me offer one bit of advice, though. Use one that has an audible signal, a click, tick-tock or whatever. A simple flashing light really doesn't cut it. Just like your eyes do not tell you how something tastes (although they may give you a good idea), they do not measure audible time. Your ears do.
Coming up with strumming patterns is a “growing pain” that everyone (well, just about everyone) goes through. You can even go through “strummer’s block,” when simply for no reason at all, you just can’t seem to get the hang of a particular song. There are times when I cannot get anything to go right and it’s necessary to take a break and “reboot.” Rhythm is one of those things that some people seem to be born with, but it is also something that you can work on to improve your own abilities.