'FL Studio' is the most user friendly music composition out there but it doesn't have "really realistic instrument sounds" in it so you will have to find some good plugins to use with FL.
'Reason' has a really good sound bank in it but the interface is not that user friendly. but if you "Rewire" reason and FL together (FL as master & reason as slave) you can use FL interface to make music and reason sound bank to get good instrument sounds.