As said in another thread, none of us can tell you what subjects are easy to do as we all find different things easy and hard. You should have a look at the thread on here on UNSW subjects nevertheless, and read what other people think about the courses they did.
No subjects are compulsory since you haven't declared your major yet, and you won't until second year. When you declare your major, then subjects become compulsory to do in order to earn your degree, I think, but you won't be barred from doing other courses in Arts that aren't in your major's field; you'll graduate later though, if you choose to do this. So, if you're somebody that doesn't know what to major in for your degree right now, don't worry, and just do the courses that kindle your interest in first year, such as a course in sociology and anthro (by the way, this is a major and not a subject!).
You can only do four subjects a semester. One course is worth 6 UOC (I have heard about there being courses of 3 UOC as well), so four courses equals 24 UOC per semester. Two semesters per year, so 48 UOC per year. You can also enroll in courses in summer school to get your units (UOC) up (though the courses you can do in summer school are quite limited). Certain majors require certain amount of UOC for you to do in order to earn a degree with a major in that field, generally nine courses (54 UOC) in that particular subject/field, but there can be more UOC required (psychology, for example, requires you to do 72 UOC relevant to psychology in order to major in it). However, doing just the courses to earn a major isn't enough to earn your bachelor; you have to do other courses to get enough UOC to earn the degree (bachelor of arts requires a minimum of 144 UOC in order to earn it, and yes, you can do two majors in a bachelor of arts), which is where free electives come in (courses you do to expand your mind, and to get enough UOC to earn your bachelor).
In a bachelor of arts programme you can enroll in courses from many areas, even in mathematics courses.
I don't know anything about commerce, but you can enroll in economic courses in the bachelor of arts programme. I think these should count as commerce courses should you eventually succeed in transferring into commerce/law, however I'm not entirely sure of this. Maybe someone else could clear this up. Anyway, read the stuff on the following link-
http://www.handbook.unsw.edu.au/undergraduate/plans/2013/ECONA13403.html
