I just lost 1.36275883 ($313), and I don't know how it happened or how to prevent it again I just opened my multi bit wallet, and the  1.36275883 that was in there is now gone. It loaded a brand new wallet, so I don't even have my old wallet with the old transactions or password. I know what my old address was. I checked it on blockchain.info, and sure enough, there is the transaction that emptied it out. 

So I can kind of make peace with losing over $300. I'm mad about it. But what I really need to know is, what happened and how do I prevent it from happening again. Can someone help with this issue?
The two most likely scenarios are that you either had malware on your computer/phone that stole your private keys and was able to spend them on your behalf, Or you used easily guessable brain wallet seed. Brain wallets use easy to remember list of words or a phrase. The problem is that if it is not done correctly, then anybody can guess random phrases and check to see if they generate a bitcoin address that has any unspent amount. There are many individuals out on the net that are attempting to brute force brain wallets. 

Depending on how much cost/hassle you are willing to put up with, there are many different ways to secure your Bitcoins. You can have an offline pc that you use to hold your coins in cold storage. Only keep daily spend amounts on your hot wallet (the coins on your phone or computer that you are going to use in daily transactions and can afford to lose). You can buy a dedicated device that handles all the security of your coins. These are called hardware wallets. A popular one is called Trezor. With a hardware wallet you are able to store your Bitcoins in a safe offline device. And when you need to spend them, you hook it up to your computer and use it to verify how much you want to spend and to were. The device keeps you secure even if your machine is infected with malware since it never lets the private keys leave the hardware wallet (you steal those you can spend your coins). 

A low cost way to have cold storage is creating a paper wallet. A paper wallet is a print out of your private keys, commonly they also include a qr code version of your keys so you can quickly import it into a phone or computer wallet without having to type out the long keys. Just make sure you keep the paper in a safe place and you treat it like cash