I would read through this section.
http
/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/
Basically when you set up a tank you have 2 options 1 a fish in cycle which uses the fish poo to create the ammonia source to start off the cycle that will establish the tank for life. The problems with this is there are fish being exposed to a lot of toxins which can easily kill them or at least make them ill which both outcomes are bad but then again you can control this cycle with lots of water changes each day for about 6 weeks.
The second way is the fishless cycle which is where you get pure ammonia from a hardware store and add a small amount each day. The ammonia is the poison created by fish poo but because there is no fish to poo you use the pure chemical to avoid putting fish in what is essentially toxic water.
So basically the choices are
controlled Fish in cycle = fish now but lots of water changes and constant risk of death and disease
OR
Fishless cycle = fish in about 6 weeks so a bit of a wait but 0 deaths and 100% safe tank to add fish to when its ready and rather than constant water changes you add a few drops of ammonia each day.
OR
uncontrolled Fish in cycle = adding the fish after no preparation and leaving the tank to stand without adding any source of ammonia to start the cycle off for a few weeks and then add fish and then they die a few days/weeks later - repeat and start again
I would do the fishless its what I did when I started and its the least stressful
Also a note on the fish you mentioned, the common fish that are kept in tanks are actually becoming some of the weakest fish around as they are bred in such mass numbers that their genes are just really bad and they are just producing really sickly fish now a days and neons and guppies are often the worst affected by this but you do get very good healthy strains of guppies but just be careful where you get them from. Also with the catfish you said about there are thousands and thousands of catfish but I would guess you are talking about either - common plecs or cory catfish google those two names and tell me which - if its the cories go for it but if its the plecs do not get a common one they are sold very often to small tanks and in reality they get to between 12-24 inches (1-2 feet) and there are so many better options than them for small tanks.
I know all of the above is quite daunting but its so important to get this part of the hobby right for your sake, the fishes sake and your wallets sake. Sorry if any of its seemed forceful or shouty but its meant with the best intentions to help you so please take note of what I have said
Wills