You could use either gravel or sand (neons don't forage in the substrate like goldfish do, so they're not fussed either way) but if you choose sand remember it must be stirred regularly to prevent it becoming compacted and harbouring nasty bacteria. Gravel doesn't have to be stirred and is generally easier to maintain, but doesn't look quite as 'tropical' as sand.
As for plants and decor, neons prefer to have some hiding places they can flee to if they're startled, so a couple of big bushy plants like cabomba, Java fern etc. would be a good idea. These can be either real or fake since the fish don't care as long as they can hide behind them! (Personally, I like real plants placed around a nice piece of driftwood or some rocks for a natural look). If you have insufficient hiding places in the tank you may find the neons don't come out much, but if you have several hiding places they come out more into open water because they feel confident knowing that they can hide if need be!
Temp: neons are happy at a temp anywhere between 76F and 82F. Mine seem to like 79-80F best. You will need a heater and a thermometer.
Other things: neons are very sensitive to poor water quality so don't try to cycle a tank with them - they are likely to keel over at the first hint of ammonia. Cycle the tank with something a bit hardier, like guppies or danios, or do a fishless cycle. They also prefer an acid-neutral pH and soft water, although mine seem quite happy in a pH of 8-8.2 and bullet-hard water! They also like plenty of aeration and water movement. They will eat most things; mine prefer tropical flake (crushed finely) or micro pellets, and they quite enjoy a bit of live food occasionally.
They are lovely little fish - I wouldn't swap mine for the world!