Follow the directions located here to germinate your cannabis seeds. After the seeds have germinated and the root is about a quarter of an inch long, place the seed, root down, on your grow cube or media that is in your hydroponic cups.
The standard thing to do is use a rockwool cube to start the seed on and surround it with whatever media you use. Keep the rockwool cube so that the bottom 70% or so is submerged when the water is at max, but keep the top part above the water so that the seed is never submerged.
There are various types of media available to grow in. Rockwool is one, but there are many others. Media simply means the substance that the roots grow in. Since you aren't using soil in a hydroponic garden, you need a substitute.
The media will provide no nutrition, it is just a support for the roots. All nutrition comes from the hydroponic solution. Do some online searching or refer to whatever book you have about hydroponics if you want to learn more about media used for hydroponic purposes.
