OKAY - it is time to set up your decorative tank. Put your floor surface down, add your decoration (remember to wash or soak) and plant and fill the container 3/4 full with cool water. Let it stand a couple of hours to adjust to room temperature and you are ready to add your fish. Is that simple enough?
As far as selecting fish, remember you don't have a lot of water to work with. You don't want to overload the tank. I would start with two small compatible fish. I know some people laugh at the standard goldfish but they are beautiful and easy to keep. I started with a couple of betas. The males are outstanding and come in an unbelievable choice of colors. The drawback is you can't put two males together or they will fight to the death. One male and one female is a good start. I also started with paradise fish. While not as beautiful as the betas, they are very hardy and can live in just about any type of environment. They may be good for the person who might not be as dedicated as they should be to cleaning the tank. Right now I have fancy guppies. They are small, colorful and cohabit nicely with each other. I have 4 (2 male, 2 female) in a rather small tank and they seem happy. They are, however, constantly having babies which isn't a problem if you have someone to take them off your hands if you don't want to grow them out. I don't mean a couple of babies...they seem to be reproducing every 6 to 8 weeks with dozens of offspring each time. If you don't remove the babies right away, mom and dad have them for dinner. It is a bit challenging but fun if you have the patience. The babies aren't a problem for me as my husband takes them to his showroom to grow out and eventually put in tanks as 'fillers' as they are very colorful.
So, now you have selected your fish and a couple of appropriate choices of food. Take them home and add some of the water that has been sitting in the tank into the receptacle the fish came in, let it sit for an hour and into the tank they go. If you purchased healthy fish, you are done. Feed the fish small amounts twice a day. Don't let old food sit in the tank (I use the baster a couple of times a day but once will do), do a partial water change every couple of days and a full cleaning (take everything out and scrub) once every 7 to 10 days. Turn the light off at night to prevent algae from growing and to give the fish a rest.
There are a lot of variables that you will just learn as you go. Don't get discouraged if you lose some fish. It may not be your fault as some fish might not be healthy when purchased. Don't be afraid to ask questions at your local aquarium store. If they know you are a beginner they will help make your decisions easier. One note of caution: If you have cats as I do, be careful where you put your decorative tank. Need I say more? Good luck and have fun!