what sort of light?
how many watts?
how long is the light on for?
how tall is the tank?
Don't buy a special plant substrate, just use normal gravel.
Most aquarium plants like a bit of light and if you only have the light on for a couple of hours a day, they struggle. If the light doesn't have a high enough wattage they also struggle. Try having the tank lights on for 10-12 hours a day.
If you get lots of green algae then reduce the light by an hour a day and monitor the algae over the next 2 weeks.
If you don't get any green algae on the glass then increase the lighting period by an hour and monitor it.
If you get a small amount of algae then the lighting time is about right.
Some plants will close their leaves up when they have had sufficient light. Ambulia, Hygrophilas and a few others close their top set of leaves first, then the next set and so on down the stem. When you see this happening, wait an hour after the leaves have closed up against the stem and then turn lights off.
Some good plants to try include Ambulia, Hygrophila polysperma, H. ruba/ rubra, Elodia (during summer, but don't buy it in winter because it falls apart), Hydrilla, common Amazon sword plant, narrow Vallis, Water Sprite (Ceratopteris thalictroides/ cornuta).
The Water Sprite normally floats on the surface but can also be planted in the substrate. The other plants should be planted in the gravel.
If you add an iron based aquarium plant fertiliser, it will help most aquarium plants do well. The liquid iron based fertilisers tend to be better than the tablet forms, although you can push the tablets under the roots of plants and that works well.
I use Sera Florena liquid plant fertiliser but there are other brands too.
Do not bother adding carbon fertiliser to the tank because they are not necessary.
