Hi Kristine,
It depends on the program! If you look at the admission requirements it will usually say the minimum average required and list a time frame, for example "applicants must have a minimum 75% average in their last 10 credits". Most programs follow the last 10 credits (2 years of full-time study) but not all so I recommend double checking the requirements for the programs you are interested in. That being said, if the institution does take the last 10 credits it will depend on your situation how many credits you will need to bump your GPA (meaning if you have a lower average in your last two years you may need to take a full year of credits to bump up your GPA, versus if you have a somewhat competitive GPA but want to raise it have a greater chance of being accepted you may only have to take one or two credits, it all depends on your situation). As your more recent courses are often used to calculate your application GPA (not your cumulative GPA), the older courses will not be used depending on when you have completed them - however - all grades are visible to admissions committees not just the last 2 years.
As for recommendations for courses to take I would recommend taking as many courses you can that are listed in a faculty of social work or social justice. It would likely look better to an admission committee if you already have an intro to social work course or something like that to show that you have some education and commitment to the field (I also recommend courses that may qualify as transfer credits to other insinuations in the future).