Hello everyone!
Before I decided to make an account on a fish forum website I looked at different threads from the different sites I found. I think this site had the best and easiest advice by far. So I made an account here and I hope to be a helpful part of the community

.
Now, down to business. I am a complete newbie when it comes to aquariums and owning fish. I currently have a 1.5 gallon tank with two tiny common goldfish that I've had for a few months. These seem to be thriving and are doing really well. The tank has a filter and a light source and they get fed goldfish flakes twice a day.
I'm hoping to start a 10 or 15 gallon freshwater tropical tank soon and this is where I'm running into trouble. I've never had a tropical tank and even though I've been doing research on it there are still a couple aspects I'm a bit confused about. First of all I hear people talking about water changes and I see that they only replace 1/3 or 1/4 of the water at a time. When I clean my coldwater tank I usually empty it completely. How does the 1/3 or 1/4 water change work? And should I be doing this with my coldwater tank as well? Also, when I start my freshwater tropical tank, how do I go about cycling it?
Another thing I had questions about is the types of fish I should include in my tropical freshwater tank. I need fish that are fairly hardy since I'm pretty much a first time tank owner. I also would like a really diverse community in my tank, but I don't want to overpopulate it. I'm interested in guppies, corydora catfish, ghost shrimp (or some other type of shrimp), and possibly platys and some other oddballs like a dwarf African clawed frog, but I am open to any suggestions on what type of fish I should get. (I'm new and don't really know what I'm talking about so I also need to know if the fish I choose will even coexist in the tank and not kill each other.) I understand that I won't be able to fit all of these into one tank, I was just wondering what this lovely community suggested

.
Also, I understand that some fish have different dietary needs (?). How can I make sure that all of the different types of fish get enough food? Also, if I get shrimp and/or a frog for my tank do I need to buy them special food?
I can't think of any other questions I have at the moment and I've already asked so many questions! Thank you so much to anyone who takes the time to read this and weigh in!