I was asked to put together 10 very basic rules for young design students (i.e. age 14 - 18) and this is what I wrote. Some of it is very rudimentary and usually goes without saying, but just in case, it always helps to remember the basics...
1. Look outside of design for inspiration.
Other visual disciplines can offer a new perspective. Instead of only looking in our specific field, go to photography exhibitions, fashion shows, old movies and look at what excites you. This will help you to avoid simply copying whatever is already being done (because if you do so, whatever you make runs the risk of being old-fashioned by the time you're finished).
2. Form follows function.
Before you begin designing anything, learn as much as you can about who the design is for (or targeted towards), what it is intended to do, what it is meant to convey, and how it will be manufactured/produced. Consider treating the design like a person, and ask what the person's favorite film, color, or magazine is. The design will be shaped by these answers and the more you know, the easier it will be to get started.
3. Save versions, save layers.
As you work and revise your designs, remember to keep the various layers and early iterations, even if you have 10 versions in one day. That way, if you make a mistake or change something drastically later, you can go back to it. At the end of a project, you can get rid of these old files and clean up your files (good practice so that you can easily find things later). However, you might also find that one of those early versions of the design make a good basis for a different project, so if you have the memory, stash those early files for a little while.
Have a reason for every design element. Use as few typefaces, lines, shapes, frames, colors, even case (i.e. upper and lower), as possible. This doesn't always mean going for a minimal approach, it just means understanding what everything you're doing is conveying. In a ridiculous number of projects, three colors are all you'll ever need, and two of them are already chosen (black and white), so you usually only need to find one color. In business, that color is usually blue or red.
5. Use a grid.
Be aware of how every element relates to every other element. Grids are obviously essential when you're laying out a book, magazine, or web page, but there is also an invisible grid of alignment linking every visual element around us. When you break the grid and allow elements to tilt or straddle two columns, do it aggressively so that it is clear that it is a conscious choice.
6. Make mistakes.
Allow yourself to try things out, see what works that you didn't expect to work, give your eyes new things to look at in your own work. If you do like your mistakes, then make it a big enough "mistake" that it is clear that you did it on purpose. E.g. If you use a broken line, break it repeatedly. If you angle an image, make it more than 5º, if your colors clash, make them clash a LOT, etc.
Discuss and share your initial designs. Even if you don't use any of their input, simply talking about the options and explaining your work will spark new directions for you. Know your own feelings about the design and be clear with yourself about what works for you and what doesn't. You don't need to argue about the work, you can just take on their input and use what is helpful to you.
8. Don't stretch a font.
Typeface design is a very skilled, time-consuming job, so you don't want to break them. If you want a headline to be taller and thinner, look for the condensed version of a font (or conversely, the expanded version for wider looks). While you may not think that anyone can see when the weight of a descender is out-of-balance, everyone can see it. They might not know what they see that makes the design look cheap and untrustworthy, but on a subliminal level, they see it.
9. Track your time.
Simply write down how much time you spend on each project each day. Be aware of how long it takes you to design something, even if you're not charging anyone by the hour. Knowing what you're capable of will help you in the future, when you're choosing what kind of project to focus on, what kind of work to look for, how to get the most out of your skillset, and where you need to strengthen your focus.
10. Share knowledge.
Nearly everything I learned, I learned from working friends and colleagues, not even necessarily on projects together, but sometimes just because they recommended a website or a book, or simply from working in the same room together and talking about how we do things. It could be as simple as learning a keyboard shortcut to do something (which, incidentally, you should try to learn now because it will save your wrists in the future, as will learning to get comfortable with using a mouse with your left hand - another thing a colleague suggested), or as big as learning about a new design style which could transform your career. Go to free classes, talks, etc, and read about design for fun. Get books out of the library, put up posters, keep packaging, immerse yourself in designs you like and let them become part of your life.