java fern is a good plant to start with. it's easy to maintain and needs little light and attention. the guppys will also like to pick stuff off of it, but shouldn't harm it.
most fish don't mess with java fern, as there is a bad-tasting toxin in the leaves. Don't worry, it won't heart you're fish.
you should try to work with easier species first. Try green cabomba, or elodea plants. Since you're keeping guppies, you might look into some plants that could provide them some shade as they spend a lot of the time at the top of the tank. Vallisneria grows long leaves that cover the surface. You could also try floating plants like water lettuce as long as you don't have a hood (thus having adequate ventallation at the top of your tank).