Hygrophilia Polysperma makes an excellent first plant. If you are worried about the possibility of snails, then Hygro is hardy enough to be dipped in a weak bleach solution for a minute or two, killing any potential nasties. Just make sure you rinse the plants thoroughly before planting.
Here is a list of easy plants that Llj has compiled:
Foreground
java moss
Christmas moss
Willow moss
Marsilea quadrifolia
Hemianthus micranthemoides (can grow towards the light or behave and stay a lovely little bush). Mine behave.
Hemianthus callitrichoides (same as HM)
Small cryptocorynes (parva, lutea, lucens, etc)
Anubia petite nana
Midground
Mid-sized crypts (wendtii, others, excellent plant)
Barclaya longifolia (can get big, can also behave)
Nymphaea species (must be taught to behave, otherwise a background plant)
Anubias (barterii, coffeefolia, barterii var nana, others)
Anubia gracilis (does better emersed rather than submerged)
Java fern
Bolbitis heudelotii (when small a midground)
Background plants
Most hygrophile species
rotala rotundifolia
Egeria densa
Egeria najas
Limnophila sessilifolia and indica and aquatica
Hydrocotyle leucocephala (Brazilian pennywort)
Nymphaea species
Amazon swords
Bacopa species
Bolbitis heudlotii (when full grown, a very slow grower)
Alternantera reineckii (striking plant, can do low-light with CO2)
Vallisneria (americana, asiatica, spiralis)
Some Sagittaria species
Dave.