Hey, you might want to list your location in your profile so local people can provide better specific advice to you. You can do that by going to the 'user control panel' at the top right of the window.
Now, about substrates, many different types of substrate are ok to use but as the others say some types can affect pH (acidity/alkalinity) of the water so it is good to research what you want and the type of fish you are keeping. Tetras generally are ok with a wider pH range that is on the acidic side so take care with shells which can raise the pH too much for the fish to thrive. When stocking your tank with more inhabitants, this is something to consider.
You will find a general trend in substrates, dark colours tend to show off the fish's colour as well as help the green colour on any plants to show up. Fine substrates, like sand, are good for plants to spread out good roots. Also, fine smooth substrates are good for bottom feeding fishes that enjoy sifting through the gravel. In some set ups light quartz type sand is used since this is what the fish would have in their natural habitat. You can go online to a number of good sites to get inspiration for how you want your tank to look. There are limitless options.