Hey there and welcome to Aquarium Advice! 50 gallons is a great size to start with. Before you add fish, you have to cycle the tank. If you look up nitrogen cycle and you'll find out how to do it. There are two ways of cycling: Fish in or fish less. Fish in cycling is where you use a hardy kind of fish to produce ammonia to feed and grow beneficial bacteria (BB) this really stresses the fish though and I wouldn't recommend it because the toxins in the water (nitrite, nitrate and ammonia) are poisonous enough to kill the fish in extreme levels. Fish less cycling means that you have an empty tank, except for your plants, driftwood and decorations, and you add a source of ammonia for the BB to break down and eat. Once the cycle is complete and there is no ammonia or nitrite in the water you will be able to add fish. A good filter is the Aquaclear 70. It should give you enough filtration capacity for a good stock. A good heater is the Eheim Jäger. Ideally, you want 5 watts of heat per gallon, so in a 50 gallon you want a heater with 250 watts. If you want t plant the tank with some god plants, I would recommend some easy low light plants like anubias, java fern, java moss, bulbs, dwarf sag, and anacharis. these are all easy, very non demanding plants. You do want some good, strong lighting for plants and most people recommend LED
's are the best for plants IMO
. I don't know any cheap lights off the top of my head, but I'm sure other people can help you with that. Are you planning on doing some big, main fish like angelfish and stocking around those or doing several groups of smaller fish? Will you be planting this tank? Some good options for this tank would be angelfish and some schooling fish, like tetras. For the substrate you need to decide between gravel and sand. I would personally not use gravel because if you ever want bottom feeders most require sand because gravel would hurt their mouths. For bottom feeders in this tank, you have a lot of options. You can do cory catfish, a school of about 12 or 14. Another good option would be kuhli loaches, but these need to be kept in groups as well. A group of 12 kuhli loaches would look awesome in this tank.