Advice on what to do before applying to Grad School in CS I’m a graduate of Carnegie Mellon, B.S. Computer Science. I am contemplating doing a masters in something Computer Science-related as well, but my GPA is a bit on the low side (barely above a 3.0). I went through a rough patch during my time at CMU, and in many of my core CS classes I struggled a bit, oftentimes not getting anything better than a C. I also found it quite tough to find a internship/job in the tech industry in spite of doing all the interview preparation I could possibly think of. I eventually ended up getting a full-time software job at a corporate and investment bank, where I currently work. I thought I would take a break from school and get experience working for at least a couple of years before deciding what to do further.
Although I don’t miss the stress/anxiety that I went through in my undergrad years, I do kind of miss some of the challenging material I got exposed to in my classes (especially those related to algorithms, complexity and machine learning), and I feel like I want to contribute something to the world of Computer Science, as crazy as it might seem. In particular, I am interested in applications of Computer Science in Economics, as well as in addressing the issue of Climate Change. Since my time at CMU, I have taken a really hard look at myself and what went wrong back then, and am determined to change myself for the better. Right now, I feel convinced that I had the wrong mindset and simply made a series of poor decisions that prevented me from doing well, learning, growing and getting opportunities.
Given the position I am in right now, I know it is really difficult to get into grad school with the grades I have, especially after reading a few forums online (Need help and advice concerning how to apply and look for grad schools and How do you get a bad transcript past Ph.D. admissions?) and discussing with a few people. However, I do feel that I am still interested in Computer Science, and would consider having a research job in the future (apart from being a senior software engineer), either in academia or industry. Does anyone have any advice on what I should do before I even bother applying for a masters to advance in my career path? Should I take online classes as well? I know for a fact that I need recommendations from research professors and quite possibly even a published paper on a research project, but I’m not even sure where to start or how to get there, given where I work right now. Any advice would be greatly appreciated - thank you!
It's good that you have some sort of (specific) idea as to what you may want to do.  I'm a potential applicant for grad school in CS myself, so take this with a grain of salt, but...
The fact that you went to a top CS school like CMU will carry some weight.  A 3.0 from CMU will definitely carry more weight than the same GPA from a lesser-name school (maybe even a little higher than 3.0).  However, a 3.0 GPA is still low, regardless of where it comes from, so you will still need something to show for that.  As you mentioned, you'll need recommendations that can speak to your abilities and convince departments that you are much better than what happened in your undergrad.  My advice would be if you have some sort of connections with a few professors from undergrad, feel free to reach out for advice and see where you can go from there, maybe even see if you can get recommendations from them.  In addition, without focusing too much or being too negative, explain in your statement of purpose how you recovered from your earlier issues in undergraduate, especially if it was because of an extenuating circumstance(s).
As far as research experience goes, you could see if there are any options back at Carnegie Mellon, although options may be more limited there, especially if professors can only afford to bring on enrolled students.  Even if not, they may be able to point you to other opportunities that could help you get where you need to go.  For CS + economics, there are also opportunities at other institutions or even in industry if you look around.  You don't necessarily need a publication (you may if you want to get your PhD at Carnegie, Stanford, etc. but others will take students without publications).