Requirements
Shinigami can go Shikai if they complete all the requirements for it, and activate it:
Name your Zanpakuto- This can be done with an ordinary anvil. Name your sword whatever you want, but be warned. Once you speak to it with that name, you cannot go Shikai with a differently named sword. Unless you do /resettype.
Speak with the Sword- By pressing right-click while sneaking, you can talk with your zanpakuto. This sets its name, and allows you to see what points you have accrued.
Gain 400 Hollow Points- Each Hollow you kill with your zanpakuto will give you points in different categories. Each Hollow gives around 5 points in different categories. You can use the Hollow Tome to decide which ones you want to kill. The counter stops when you get to 400, and your zanpakuto's type is set in whatever category has a majority.
Have over 200 Spiritual Energy- The final requirement is to have over 200 spiritual energy. But that's just the cap. You can have less if you want to activate it, but you must have reached 200 total to be allowed to go Shikai.
Hold Right-Click- To activate Shikai, simply hold right-click for a while. A ring of particles should appear around you if you are able to go into Shikai. On releasing, your sword will transform and be much awesomer.
Types
Each zanpakuto type is different, and allows you to do many things. The different types are as follows:
Fire- Your basic fire sword. Catches mobs on fire, useable as flint and steel, gives you fire resistance. Some very cool textures. Fire Bankai is where the fun will begin.
Ice- This Shikai has a few cool features. Slows down mobs that you hit, and has increased damage in the snow. Also, my favorite, freezes the water underneath you for a quick getaway, or personal ice rink.
Poison- Not much to be said about this one. Poisons your enemies when you hit them, but also poisons those who hit you while you have this sword out. If anyone has cool poison Shikai ideas, let's hear them.
Heal- The opposite of poisoning. If you hold right-click for a while, you will get absorption if you are in a tight spot. Also heals you when your health gets low, and can revert zombies to villagers if you right-click them. Doesn't always work though…
Earth- A strong sword with good knockback. Basically has increased damage and knockback, and is more powerful on land. Also looking for community input for this one.
Wind- One of the most fun Shikai, in my opinion. Gives you a massive jump boost and protects you from fall damage. Hits mobs into the air, so you can juggle them if you master it.
Light- Another slightly less impressive sword, sorry to say. It is much more powerful in the day time, and it gives you invisibility for a while if you hold down right-click.
Dark- The opposite of light. Much more powerful at night, and gives enemies blindness when you hit them. But dark Bankai should be fun…
Lightning- A mixture of Light and Wind type. Sometimes will hit enemies with lightning bolts when you hit them. For a fun time, hold down right-click for a while. Paralyzing aspect in the works.
Water- A mixture of Ice and Fire type. Hold down right-click to make it rain. This sword is more powerful when you are in the rain, or in water. Allows you to swim as fast as a boat, breathe underwater, and see better underwater. One of my personal favorites.
Lunar- A mixture of Light and Dark. Very WIP at the moment, so I would not suggest trying for it right now. Gives you a speed and strength boost, and fires a buggy Getsugatenshou if you right-click. Very terrain-killing
Normal- If you have no majority at all, your zanpakuto will become Normal type. This means a very basic sword. Has increased damage from the zanpakuto, makes you go a bit faster and gives a bit more defense when blocking. How normal.