I'm entering McMaster as a transfer student, I'm going into first year with 9 units (3 classes) so technically I only need to take 7 classes (21 units) rather than 10 classes (30 units).

However I want to take 10 classes, I was thinking of taking 3 second year electives plus the 7 first year classes that I need. I plan on majoring in Communications, but the 2nd year CMST classes require admission into the program first so I want to take some of my 2nd year electives now so I can just focus on my required core Communications courses the following year.

So, how do I know which second year electives I can take in first year that will go towards a Communications degree in the long run? Is there a list anywhere that shows what second year electives you can take? Or do all 2nd year humanities courses require admission into its respective program first?

Yeah, most Humanities 2nd year courses will need you to be registered into the program before you can take it. There are some Humanities 2nd year courses that don't require you to be registered to the program but you can take them as long as you have the required 1st year courses. Since you're only going into first year, you are limited to taking 1st year courses. Here are the course listings for Communitions Studies for your reference: http://130.113.236.37/CALENDAR/current/pg2377.html

If you just want to take electives, you look up the program requirements and see if there are any restrictions. You don't need to be registered in the program to see the requirements that need to be completed - they're all in the course calendar.
Keep in mind that the link posted below is for this past year; you should use the one for the 2014-2015 school year. It's on the registrar's website and can also be found through google.

Here is the current one: http://academiccalendars.rom cmaste...ourse_fil ter

I believe there are many upper year History and Religious Studies courses that don't require any prerequisites (not even registration above level 1)