Congratulations! So you've decided to create a horcrux. However, it's not as straightforward as it may first seem. Here are just a few things you need to know before taking that first, irreversible step.
1. To create a horcrux, you must choose an object that will the piece of soul ripped from your body. Inanimate objects are preferable, but living creatures have been used before.
2. Horcruxes are created for the sole intention of attaining immortality. You can not be fully killed until your horcrux has been destroyed. Very few things can destroy a horcrux. These include basilisk venom and fiendfyre, while the only way to destroy a living horcrux is to kill it. Either an item imbued with basilisk venom or the killing curse (Avada Kedavra) will work.
3. To create a horcrux, you first have to intentionally commit murder. When choosing your victim, it's preferable, though not essential, to make this an important murder.
4. Lord Voldemort created seven horcruxes, not the six he intended to make, being the only person we know of so far to create two living horcruxes. When he tried to kill Harry Potter on Halloween 1981, Lily Potter's sacrifice for her son caused the killing curse to bounce off of Harry and back onto Voldemort, killing him and splitting his soul. It attached itself to the only living thing in the room - one year old Harry. Voldemort wasn't completely dead however.
"...If the maker was later killed, he or she would continue to exist in a non-corporeal form, although there were methods of regaining a physical body."
5. Horcruxes are said to be the opposite of a person. If a person's container, their body, were to be destroyed, there would be no harm to the soul, whereas the fragment of soul contained within the horcrux cannot survive without it's host, it's container. When the piece of soul realises it is about to be destroyed, it will fight back for it's very existence.
6. Items that have been previously used as horcruxes include Tom Riddle's diary, the lost diadem of Rowena Ravenclaw, Salazar Slytherin's locket, Helga Hufflepuff's cup, the resurrection stone (see Deathly Hallows), Nagini the snake, and Harry Potter. Each time a horcrux was created, Lord Voldemort's soul was split into smaller, even weaker pieces.
7. Finally, when choosing your own horcrux, something that cannot easily be found is a good start. Examples include: a Nokia 3310 (a vintage muggle contraption used for communicating with one another; said to be stronger than a house brick); an ordinary pebble from the beach; a teaspoon or a sock - these are usually lost to the netherworld after using just once; and finally a simple knut, or galleon.