How to capture source IP address(clientIP) on F5 LTM .

The generic answer is every system is different so it really depends on what kind of data your looking to log. If you want lots of logs you could look into combining a remote syslog server with either a Request Logging Profile or High Speed Logging, there some good reference posts on those already.

You could also take the two I listed above and combine them for something like this:

when CLIENT_ACCEPTED {
   set userlogged 0
}

when HTTP_REQUEST {
    if { $userlogged == 0 } {
        log local0. "clientIP:[IP::client_addr] accessed [HTTP::host][HTTP::uri]"
        set userlogged 1
    }
}

The first part will initialize the userlogged variable when the client first connects, and then once the HTTP_REQUEST event is triggered it will log only the first request by the user as userlogged is then set to 1.