The following is about design decisions, but don't be fooled — we all make them. Decisions are based on design, as long as they are purposeful. They can be in food preparation, home decoration, word use, or any such thing.
You should learn about design principles, for there are opportunities to apply them. Most importantly, design can never be limited to one specific aspect of a given thing; it has to take into account every possible way in which one might experience the design piece. One common fallacy is to assume that only perfectly able humans will be using a space. To avoid the exclusion of anyone who might have been forgotten by the designer, requirements are put in place to make public areas accessible for everyone. As personal design decisions are not subject to regulation, you need to be particularly mindful of all use- and abuse cases. Take as an example the web designer who will not publish a page before it is compatible with all major web browsers, as well as accessibility devices such as braille readers. After all, you will always create a sensible experience, being experienced in non-obvious ways. When you are clear about every aspect that needs to be designed, make sure to set sensible — i.e. appropriate — defaults. Far too often, we assume that the thing is only a temporary solution, and future humans would apply their preferences to modify the item. It is the wrong choice! Solutions that are good enough will never be fixed, so be strict with yourself to not settle for inferior solutions or the illusion of choice. Should you buy that beautiful clock if its ticking is going to annoy a member of the family for the next ten years? Should you install that cheap Norman door if hundreds of people are going to bump into it because of a misleading handle? Should you address a mixed-gender group as guys if most people don't think of a woman as a guy?
Please make decisions like a designer and be sensible about the execution.
Update 2017-05-30: I have become aware of a wonderful alternative to guys when addressing a group of people: Cats. I wholeheartedly endorse the usage of cats.