Patterning is probably the toughest part of beadcraft to do well. Our brains are hard-wired to seek out patterns in the world around us, and to organize data into small bits that are easy to recognize. This is the reason, for example, that things like phone numbers and social security numbers are broken down into small segments, separated by dashes. Simply put, they are easier to remember that way.
One big mistake that a novice beader will often make is simply to use "whatever is on hand" and attempt to design something workable from that. This CAN produce beautiful results (especially if supplies are plentiful), but often results in designs that are haphazard in their appearance. Randomness. Don't take that chance. Spend some time and learn the art of patterning.
Once you embrace the notion of the pattern, you can (regardless of your skill level) get downright lost in it, experimenting with different configurations (sometimes for hours or even days at a time) until they hit upon a design that is truly compelling. This is a fine use of your time, and great practice, but don't make the mistake of thinking that designs and patterns have to be complex, either! (although as a general rule of thumb, the overall complexity of your designs will tend to increase as you gain more experience). One of the all time classics is the traditional strand of pearls, and as designs go, it doesn't get any simpler than that!
For some reason, a fairly significant number of people who are new to the hobby find themselves at least slightly intimidated by the notion of patterning. This is unfortunate, but in some ways, understandable. Still, if you stay with it, you'll find that as your skill level increases, your fear and misgivings of patterning will diminish, and almost before you know it, you'll be off and creating on your own. If you're not there yet, however, don't be afraid to look around (online or off!) for a little help! And there's tons of help to be had! Offline, there are books galore on the subject, and online, there are thousands upon thousands of sites devoted to all aspects of bead craft and beading ideas. Explore for yourself and see! Almost before you know it, and even if you start with a design you found elsewhere, you'll find yourself tweaking it and making it your own, and you're on your way!
The mechanics of bead craft are simple. The patterning...less so. It's a learned skill and it can be a little intimidating. Do yourself a favor though, and if you're not coming up with ideas on your own, or if you're a little intimidated by the though of having to, check out on or offline resources for inspirations for beading ideas. You'll be glad you did, and from there, well on your way to mastering the fine art of patterning!