i would still suggest to look into my suggestions moving forward. continue watching those videos not just for the sake of learning, but to see what options people use so you can use your own research for your decisions. you'll get the same results here on R2R where some input is straightforward whilst others may seem farfetched or confusing. so take your time, and no not rush into anything especially now when all you have is just a tank.
filtration - will you be plumbing your own tank? if so, you must take into account plumbing, the tools for it, etc. if not, will you be running a HOB or canister filter? or will you use a HOB skimmer? are you going to run a sump?
heating - your fish and corals will need this.
wavepump - needed to move water in the tank. stagnant water is "bad" water.
additionally, as someone already mentioned, you'll need sand/rocks. this will be easy to source/obtain. so don't worry about this as well as lighting for now.
edit: did the tank come with a stand?
a healthy budget for that 45G could be anywhere from 0$-2,000$+ to get it running. of course it can be less, or more depending how you obtain the equipment (free, used, new, etc). the original 200$ you mentioned would likely only get you a used skimmer with some left over money for whatever else you'll save up for next.