Hey its very simple,

You have to perform in coding level of testing, it is called
white box testing where as you have perform in behavior of
application like., functional level, it is called block box
testing.

It's totally on your experience,White box testing is 
tested when you have good knowldge of Coding
(Develpoment),either you have to with black box testing.Its 
never happens that your senior tell you to go for white 
box.So don't be bothered for that.

Unit and Integration Testing will be come under whitebox 
testing[This should be take care by developers].Coming to 
system testing will be treated as block box testing.

white box testing would be done by the developers since it 
demands coding knowledge. so white box testing techniques 
are branch path testing, basis path testing. As far as 
black box testing is concerned, it is mostly done by the 
functional testers. Those who have very good understanding 
on the domain they would be testing. Black box is like a 
non-transparent box, where the tester only knows what to 
input and what to get as an output and whatever happens 
inbetween is a black box for the tester.