You can heal yourself a multitude of ways in the game. I presume you mean heal your HP, but in case you meant something else, I'll be as thorough as possible.
1) To Heal Your HP (Health Points/Hit Points) Obtain and Eat Fruit: Many fruit can heal your HP. Hovering over an item in your inventory is the absolute best way to see how the item can help you. Apples (Red Apples) are the most common fruit that can help your HP. Other items include, but are not limited to; pomegranates, orange/red bananas.
Use a Spell/Have a Spell Put on You: Depending on your characters abilities, you may be able to cast a spell on yourself which will increase your HP. Whether you have the ability or not, someone nearby (especially if you're in a party) can cast a spell that may help your HP.
Stay Out of Conflict: One of the best ways to regain your HP is by remaining out of a fight until your HP goes back up.
2) To Heal Your MP (Mana Points) Obtain and Eat Fruit:Again, hover over items in your inventory! There are many items you'll discover that can help your HP, MP, and SP. The most common fruit that can help your MP are Green Apples. Use a Spell/Have a Spell Cast on You: See answer two to part one above. You may be able to cast a spell on yourself to heal your MP.
Stay Out of Conflict: See answer three to part one above. Remain out of a fight! Your points will slowly creep back up.
3) To Heal Your SP (Stamina Points) Review the answers above. Fruit, spells, and keeping out of conflict are all extremely helpful in helping your points!
Need healed in another way? Sometimes a monster can cast something on you. Search for cures within the game to remedy the situation.