How do you connect to the Bitcoin Client?
You need to run either Bitcoin-Qt or bitcoind on your system.  Create a bitcoin.conf file containing the following lines:

rpcuser=your-userid
rpcpassword=your-password
server=1


You can then connect to your bitcoin node by specifying address 127.0.0.1 and port 8332.  Use the userid and password you specified in the bitcoin.conf file.

While it is possible to open port 8332 to allow remote users to connect to the local node, this is not a good idea for security reasons because it allows access to your wallet.  Instead, your users would communicate with your application and your application would then make the necessary RPC calls to the local node.