Hi, a good sized tank to start with! I've made a list of fish which will be good in a tank that size (not all at the same time, of course), which you might find useful: http

/aquariumadventure.wordpress.com/species-for-a-60-litre-aquariums/
For planted, I prefer to have 1 cm of substrate at the front, going up to 10 cm at the back, which is on average 6 cm. So, that will be 61*31*6/1000 = 11.3 litres of substrate. At this point, you either need to find the conversion for litres to kg. For unplanted, I prefer 2-3 cm throughout, so around 5 litres would do well. I recommend that you reconsider gravel in favour of sand. It will give you more options in terms of fish and critters.
Please fishless cycle your filter, it often proves to be less headache and is safer for the fish: http

/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/