Getting application data through libpcap

Hello all !!
I am trying on my hands on libpcap - the packet sniffing library.
Through the tutorials, I got to know how to get the data from Ethernet, IP and TCP headers - but I want to read HTTP data now (or any other application data). I know the first step is using a filter, which I did. But what next ? Where would I get the packet structure of HTTP ?

In short, I need advice in reading the application level content in the network packets( from TCP payload) to a file. Please do tell me, if I am trying something infeasible...because I googled a lot for this, but no success :(

Thanks !