St. Johns Wort is a very simple unassuming plant. It's very easy to grow up to zone 5 and does well in full sun or partial shade. It can be used fresh or dried and all aerial parts are used and should be harvested in summer.
It has long been used to treat anxiety, irritability, depression, emotional instability and some conditions where there is nerve pain. It's easy to use in teas and tinctures but doesn't mix well with many other herbs, so research anything you intend to add with it. I much prefer using single herbs whenever possible anyway tho you can add other complimenting medicinals. But remember also that many things will counteract or change other things.
Topically, it can be use in balms and salves for burns, inflammation and muscle aches. You can make a wonderfully relaxing massage oil by adding St. Johns Wort oil (which is very red) to other oils such as almond oil.
St. Johns Wort can also be used in soaps and candles as aromatherapy, with similar effects. If you can't make or get soap or candles, you can easily take some fresh St. Johns Wort or essential oil, and add it to some simple Epson salt or sea salt. For fresh, bruise gently and add to salts in a container with a tight lid and room for the salts to move around. Store in a cool dry place out of sunlight and each day shake or mix with wooden spoon. After 7 to 10 days it's ready for use. Just throw a handful under running water when preparing a bath. If you use fresh and don't want leaves in your bath, just wrap them loosely in cheesecloth or add them to a muslin bag before adding to the salts. In a gallon size jar of salts I add around 3 large handfuls of St. Johns Wort. The salt will desiccate the herbs a bit and I quite like them in the tub.
St. Johns flowers are also gathered and used by natural dyers for yellow and gold.