Ftp? Really? With username and password? Unless your server is really well configured, FTP is a security hole waiting to happen.
But... OK... to answer your question; there is no built in FTP client API in iOS. You'll either need to roll your own (hard, but the protocol is well documented) or use a client library (maybe easy, maybe not).
I'd start here (yes, a google search, but not meant to be flippant) and review the various libraries to see which fits your needs better.
Note that Omni's frameworks show up on that page. They write good code.
However, the best solution would be to use HTTP to communicate between your app and a server. HTTP support is built in, HTTP is ubiquitous, and it is much easier to configure a web server to be secure.