To create your own wallet, you should start by downloading the bitcoin client and running it: from there you can make as many new addresses you want which will be stored in your wallet.
From mybitcoin you can then send BTC's to your new address.

For securing your wallet, take a look at the wiki.

As for trusting mybitcoin with all your money: that is up to you to decide. Personally, I wouldn't for a couple of reasons:

• I trust myself more to handle my data/wallet safely than I do a third-party.
• Mybitcoin holds accounts for different users, making it a nicer target for crackers: if they manage to break in, they will gain access to a lot of different wallets, not just yours.

If you don't store too much there it shouldn't be much of a problem, but if you put all your life savings there and it ends up lost, then you can only blame yourself for putting all your eggs in the same basket.

ok so i transfer to my wallet..and then back it up...i wont have to make a new back up everytime i make a transaction right?

and after a transaction it made a new address? could i use the old receiving address again?

Your wallet doesn't technically contain your bitcoins.  It only contains the keys which authorize you to use them.  There is one key per address, so you should only need to create a backup whenever you create a new address.  (I've been told that when you first create a wallet, multiple addresses are pre-made under the hood, which means that after a single backup, you're good until you've made more addresses beyond this initial amount.)

And yes, you can definitely reuse the same address as much as you want.  Addresses are basically like public boxes that anyone can see inside, learn the history of, and deposit bitcoins into, but only the person with the key can pull bitcoins out of.  Reusing a single address lets everyone else also easily determine that all the transactions into and out of that box are related by all being to or from the same person, thus decreasing anonymity.  And if all your money is stored behind a single address, then if someone else acquires your key to just that one address, then they have complete access to your money.  Keeping money in multiple addresses helps avoid this, except that a single wallet can contain many keys, so that to really be secure from key theft, it helps to have multiple wallets, each with just a few addresses in them.  But that complicates things a lot, obviously.  It's a bit of a tradeoff between increasing security and increasing convenience.  (They're both pretty high as it is, but if you want one to be even higher, then the other typically has to go down some.)

Disclaimer:  I am a bitcoin noob.  I cannot guarantee that I am completely accurate.

thanks you guys..youve all been really helpful..i think ive finally figured it out..

im not comfortable with keeping my coins on mybitcoin..