What sort of plants do you have? There are many different aquatic plants, all with different needs, and to make matters worse, non-aquatic plants are often sold as aquatic. Many plants require higher lighting levels than you get with stock lighting, light spectrum can also make a difference. Some species need a nutrient rich substrate or other special fertilizers. There are also many fish that will chew up most aquatic plants, what are you stocking?
If you're a newbie to aquatic plants, select a few hardy low-light species rather than going for whatever looks pretty at the shop. Here are a few that are practically foolproof:
Anubias, java fern and java moss all do very well with low lighting, however none of these should be completely planted in the substrate. Keep the rhizomes of anubias and java fern above the substrate at all times or attach any of these plants to rocks or bogwood. Anubias especially is a slow grower, but these have thick leaves which most fish won't touch.
Anacharis is an especially fast growing stem plant which I've found to be impossbile to kill. It can be rooted into any substrate or left to float at the top of the aquarium. It's also extremely easy to propogate from cuttings.
If you can get those growing in your tank, you might add a few more hardy species:
Cryptocorynes don't have much in the way of light requirements, though they do best with a nutrient rich substrate or root tab fertilizers. They are however sensitive to changes in water conditions, and will often melt if you move them though they'll quickly grow back so long as there is a healthy root structure intact.
Swords are also heavy root feeders without many requirements when it comes to lighting.
Vallisneria is a fast grower that propogates with runners, one of the easier beginning plants.
Marmimo moss balls are an interesting addition and don't require much lighting or other special care unless you're going for high growth.