First, realize that you need to understand yourself. If you don't know what makes you happy or relaxed, you won't make much progress. What you like may differ from what I like, so just use my examples as inspiration.
Second, trust your instincts. You don't need to be a home decorator to make a space you like, because you only have yourself to please -- although if you live with other people, decorating common areas to multiple tastes can be more challenging. You also don't have spend a ton of money on this project. Many improvements are cheap or even free.
Here are some things to consider ...
* Do colors influence your mood? They do for me, but not everyone feels that way. So for instance, the living room here at Fieldhaven is done in greens and browns, like a restful forest clearing; my home office is cheery shades of yellow and orange. Choose soothing colors in rooms for relaxation or sleep, uplifting colors in rooms for eating or working. Is one room a color you hate? Change it if you can.
* Do you have a big space or a small space? Large furniture and darker colors bring a big room into human scale. You can also subdivide it with bookcases, which is what we've done in several places. Small furniture and lighter colors help a tiny room feel bigger. If a room feels the wrong size, you're never going to get comfortable in it.
* Do you like live things? House plants, fish tanks, etc. add living energy to your home. They are soothing to watch, and nature in general is good for people. I have a whole bay window full of plants, along with birdfeeders outside near windows.
* Continuing the above idea, maximize use of windows if you have a good view of nature. If you don't have an actual view, you can substitute a poster (cheap) or virtual window (pricey but more effective). These moving murals look great and have sound options. Sometimes malls have a kiosk selling backlit nature scenes with sound and motion, usually a water feature; these are among the cheapest virtual windows that are really good. Sunlight is good for you (in moderation), especially a sunny window, but daylight lamps help too. Some of the latter take the form of panels; hang one on the wall, frame it with curtains, and it rather resembles a window.
* Does noise bother you? If so, include soundproofing measures. You can buy or make acoustic panels which are decorative as well as protective. Curtains help too. Another option is covering up unpleasant sounds with pleasant ones. There are many options to consider such as sound machines, bubble walls, tabletop fountains, and so on.
* Do you enjoy light and motion or find them distracting? I like to hang suncatchers, window prisms, windchimes, and mobiles. If it doesn't come with crystals I often add them. Rainbows dancing on the wall make me smile. Mirrors are good too.
* What kind of scents do you like? Incense, candles, aromatherapy sprays, essential oils, herbal sachets, and dreampillows are good options to create pleasant scents. Chemical fragrances, not so much. Note that you can take any throw pillow, open a seam, and stuff a sachet inside to make an aromatherapy pillow; then just close the seam.
* Include items for alerting and grounding. Alerting tools wake you up when you start to drift. Grounding tools help you connect with your body in the here-and-now. Although they seem like opposites, often the same tool can do both, because it's all about getting you out of your head. I have textured box-bottom afghan draped over the back of the couch. I touch it every time I walk past, running my hand over the bumps, which is both alerting and grounding for me. It's a great way to refocus after I leave my office.
* Relaxation tools are good. I have found that some of these actually regenerate energy. I am especially fond of certain new microfiber blankets and soft squashy pillows. Just recently I found a stuffed cat pillow with very short microfiber fur. You can buy hot/cold packs or make your own with whatever filling you like. Really, no craft skill is required: pour a bag of rice into a cotton sock, knot the end, and it's done. I have two hot socks so I can rotate them, very nice when the wind is howling and the house won't stay warm.
* Decorate with things that make you smile. It doesn't matter what they are, only that they make you happy. I use houseplants, prisms, afghans, artwork, and other things for this purpose. Do what works for you.