I've been geocaching for 12 or 13 years. I'm no power cacher, but I do have a few hundred finds. But you know what? I also have a couple hundred no finds. Some may have been missing at the time, while others were found the very next day by someone else. A few I managed after a subsequent visit, but there are a small number that elude me to this day.
Sometimes the Force is not with you. Sometimes your Cache-Fu doesn't bring you answers. Sometimes the hide is just that tricky. What you have experienced is, in a word, normal.
With that said, the answer to your underlying question is hard to pin-point, but in general, look for something out of place, and remember that GPS accuracy can be as bad as 20 to 30 ft (6 to 10m). Here are a few examples:
- The most obvious one: a too-perfect-grouping of sticks. A single stick on the ground in the woods is just a stick. Even a pile of sticks can happen at times. But a gathering of sticks, all aligned in the same direction, leaned against a stump? That probably hides a geocache.
- Rocks. Rocks do not naturally pile together. A pile of rocks has a geocache underneath.
- A pine cone in a maple tree. It might have fallen off a nearby pine tree. But then again, if there are no nearby pine trees, it might be a fake! Perhaps a cap screws off or something, and there's your geocache.
- A bird house without an opening for a bird to go into. Look for a hinged side or something, and the geocache within.
There are more I could list, but at some point I would cross the line into spoilers, and ruin the game for you. Good luck!