bitmixer.io runs a great service, but follow instructions, take your time and go slowly until you know how it works.
To add to above, OP the most important part about mixing your coins on bitmixer is the Letter_of_Guarantee, make sure before you send your coins to get mixed, you download that letter and verify the message that it is signed by the original bitmixer address and confirm that the address provided in the letter is the same one showing up at the site.Yep.
Here is a recent victim of a man in the middle (MITM) scam: https://bitcointalksearch.org/topic/httpsbitmixerio-wrong-address-in-letter-of-gaurantee-and-self-tumble-1470538
I find the easiest way to verify a message is going to blockchain.info, opening the demo wallet or your own wallet, whichever one is easier (the latter should be harder, otherwise your security isn't enough), clocking import/export, and then "more", then "Sign/Verify message".
