What do you need? Water and light.
After that, honestly, you branch off into a ton of different factors.
Some plants are floaters, or have no roots to speak of (they get nutrients from the water, through their leaves), so those don't require substrate (i.e. sand, gravel, etc.). Others are root plants, and the type of substrate can be important.
Different plants require different amounts of light to grow well.
Nutrition needs will vary, based on many things, such as ... what type of substrate you use, how much light you have, what kind of plants you have, what type of water you're using, etc.
It really depends a lot on what you want, and what you are willing to invest in terms of time, space (for equipment), and money.
The amount of lighting you decide on is probably the biggest factor in determining the other things. It will determine, among other things, what plants you can grow, and whether or not it would be prudent/necessary to inject CO2 into your water.