So I had some bitcoins at time of the fork. Do I have the other new coins as well? How can I spend, sell or buy them? Is there a multibit for bitcoin cash?
IT depends. If you own your keys, then yes, you have an equal number of BCH now. If you don't it's up to your exchange to decide if they will support the fork, and therefore, weather or not you now have an equal number of BCH.
Yes, but it's hard to trade them right now unless you had Bitcoins in an exchange that auto-balanced at the time of the fork.
There are some new wallets that support BitcoinCash, but being new, they are not trusted yet. Here's the official procedure to safely use the Electrum Cash (fork) wallet according to the non-fork creators: https://electrum.org/bcc2.txt
Even after you do that, most exchanges are not yet supporting importing BitcoinCash into their accounts. Once everyone is able to do so, the price is expected to tank. I guess we'll see...
did you have it on a wallet online or in a wallet you created with a software locally?
if its locally created, then good news. you should download Electron Cash and import the existing wallet and you shall see your BCCs.
That's a common question right now. So is, "should I sell my BTC in case it fails, and assume I will still have BCH?" The market might help you make that decision when the exchanges open BCH withdrawals and deposits.
If you had those coins on Bitfinex or Kraken then yes your account will have an equal amount of BCC. As they are on your exchange account, you can trade them for BTC and go about spending those the usual way. You can also withdraw BCC but it'd take days.