DO obey the inch-per-gallon rule until you are quite experienced
DO weekly water changes of between 10% and 50%
DO keep fish such as tetras, corys and rasboras in shoals
DO research all fish you like BEFORE buying them
DO overfilter rather than underfilter
DO perform weekly or fortnightly gravel cleans
DO use water conditioner
DO test your water regulary
DO buy your own liquid test kits for ammonia, nitrites, pH and nitrates
DO stock slowly
DO make sure all your fish will get on well
DO make use of cheap, second hand equipment (Ebay and Freecycle ftw)
DO
read this
DO use live plants
DO NOT buy fish that will outgrow your tank, even with the best intentions of upgrading in the future
DO NOT buy fish on impulse unless you know a lot about them beforehand
DO NOT buy a common, gibbiceps or sailfin plec unless you have a 6 foot tank
DO NOT take a shop's word on something - research yourself
DO NOT keep bettas in jars or small tanks (5 gallons is good)
DO NOT assume fish "grow to the size of their tank" (and if they do, it usually kills them eventually)
DO NOT keep goldfish in bowls or any tank under 100 litres for that matter
I've got loads

will add more when I think of them!