Hm; goldfish tend to be a bit messy in smaller tanks. If you wanted them, you could feasably fit two in a 20g. But comets/commons can get upwards of a foot in length, so this is not big enough for them permanently. Two "fancy" goldfish would be fine in that size of a tank. Orandas are a hardier type of fancy goldfish;if you're beginning, I would advise against bubble-eyes, pearlscales and the more fragile ones. Black moors and telescopes are good choices too, if you think they're cute (I do.)
You could get some lower-maintenance tropicals if you installed a heater; this is not hard at all, to keep at the right temperature in a normally-temperatured room. Goldfish are great fish, but are pretty messy and produce a lot of ammonia; tropicals such as tetras, platies, and swordtails are a bit cleaner in relation, and you could get more in a 20g tank with relatively less cleaning than goldies.