Make a gauze and bandage your head by left clicking the gauze in your inventory and then right clicking on your character. This will continuously heal hitpoints. Gauze is seldom available to newer players as they need make it it from the fleece of a Mouflon. This works either by domesticating it into a sheep and shearing wool, or by feeding a wild one a four-leafed clover; the wild mouflon will allow you to shear it for 1 unit of wool afterwards. Gauze works slowly, but very surely, and with no risk up to 10 HHP per piece of gauze. You can only use one bandage at a time and cannot wear any headgear while your head is bandaged. Used gauze turns to bloody rags.
Leeches heal your HHP (the middle number), but do damage to your SHP (the first number). They randomly attach to your equipment slots when walking through green swamps. This is the most available method for new players. First you need to locate a swamp. This terrain shows greeny-blue on both the map and on the minimap. There is an almost identical terrain that shows greeny-blue on the map but white on the minimap and doesn't have leeches. It is a good idea to collect leeches and store them for future use if your dwelling is not conveniently close to a marsh. When the leeches have attached to your equipment slots, they begin drinking your blood. This is what causes your soft SHP (the left number) to go down. A single leech takes about 10 SHP to bloat. If your SHP goes down to 0, your character will drop unconscious, AND you will then lose HHP! Thus you should start using leeches for healing when your SHP is maximal (i.e. equal to your HHP). SHP regenerate relatively fast, but cost hunger. Keep in mind that regenerating SHP has not been affected by the hunger-stamina fix. You will not always get a HHP back for every time you lose a SHP to leeches, so you have to be patient. Don't overdo it by covering yourself with leeches all over. Bloated leeches stay on the character, so a K.O. shouldn't prove lethal, unless you were desperate and your HHP already in single digits. Pick bloated leeches off and put them into your inventory, or a container, to save for another time. Replace them with unbloated leeches if available. Bloated leeches go back to being unbloated leeches after 8 real-time hours. Unbloated leeches do not starve. Leeches adjust quality by sucking your blood, defined by constitution value. High-quality leeches on average regain more HHP per use. Thus taking leeches previously used by high-con characters is beneficial to the player, but will decrease their quality. Bloated and unbloated leeches can of course be used for bait.
Feed a deer a carrot and it may heal you.