I've been inclined to get an aquarium for myself and been thinking about it for the last couple of days. There's only one issue, I feel like my apartment gets too cold at night and somewhat cold during the day, since it doesn't really heat up unless I open the windows(which I can't unless I'm staying in, cause it rains randomly so I can't just open the windows when I leave to work) The coldest we've been during the night is 39F and during the day it ranges from 45F - 62F, so I was wondering what should I consider or what are the right safety measures (size of tank, type of fish, type of light) I should take into account for my very first aquarium?
Freshwater needs less technique so I think it is easier to start with and less expensive as well - including the fish... The requirements to temperature are very different and I would recommend to start with fish which prefer lower temperatures in your case - e. g. Tanichthys albonubes (https://en.wikipedia.org/wiki/White_Cloud_Mountain_minnow) and Corydoras paleatus / Peppered catfish (https://en.wikipedia.org/wiki/Corydoras_paleatus). Both are easy to keep and absolutely suitable for beginners.
Get a heater larger than the recommended size for whatever tank you decide on.
Most tropical fish need temperatures in the low 70's to the mid-90's, so with your ambient temperature there's absolutely no question you will need a heater. The exception to this is if you run a temperate tank which the stocking requires much colder temperatures. These tanks typically have very few options for stocking and cannot be stocked at a comparable level to tropical fish tanks. I would not suggest a temperate tank as a first aquarium.
As far as what size aquarium, this one is subjective to the amount of space you have and your initial budget and how much work you want to do on the tank. Larger tanks offer much better stability as well as many more options for the actual fish you keep. They are more expensive to setup and initially purchase and may require much more routine maintenance. If you are going to be running a heater 24 hours a day, which your ambient temperature would suggest is a possibility, make sure to account for the costs of running the tank itself. For a quick example; running a 300W heater 24 hours a day will cost slightly more than $20 per month in electricity at 10¢ per KWH. The costs of operating larger tanks, especially with high power lighting and large heaters or chillers, can quickly become a significant and continuous expense.
Lighting should be based on whether you plan on keeping live plants, or coral in the case of saltwater tanks. Otherwise for fish only, the lighting isn't extremely important, just be aware that low quality and poor spectrum lighting can contribute to algae problems.
Type of fish is dependent on how you plan the tank. You can pick fish first and design the tank setup around those fish, or you can design a setup that is appropriate for a variety of fish. Neither is a better solution, if you know what type of fish you are most interested in, I would start there.