No matter what, if you want to have a proper set-up no matter the size you will need a bit of cash on hand.
Firstly, the aquarium itself can cost some money. I'm not too sure on prices where you live, but it's a bit expensive where I live. One thing is purchasing a kit (tank, hood, lights, filter included etc) because this can save you a little bit. Buying them all separate can run you into a lot of money. Plus, depending on the fish you may need to buy a larger filter and a heater (if any tropical fish).
Also, liquid testing kits for Ammonia, Nitrite and Nitrate, water conditioner/de-chlorinator, a gravel vacuum, a fish net, and a variety of fish food (depending on what type) is essential for keeping healthy fish.
It's not all a rich man's hobby, but you need to have some money. The most important thing to have when keeping fish is knowledge and research. Make sure you thoroughly research your fish before even consider purchasing one, meeting all of it's needs. Also, please fishless cycle your tank before getting fish and good luck.