Welcome to the hobby!
If you're planning to have bottom dwelling fish like corydoras, then they do need sand rather than gravel, sand fine enough and not sharp, since they will filter feed through the sand and expel it through their gills.
A lot of people in the hobby have switched to sand rather than gravel since it's also easier to keep clean than gravel is, but it's still your choice! There are many many debates about gravel vs sand here... if you go to the search bar and type "gravel or sand", you'll find many many threads where people talk about the pros and cons. Worth reading through a few of them so you can choose what suits your plans!
Oh, edited to say if you're planning a more river type set up, with strong flow and fish like certain loaches, gobies or other strong current river dwellers, then a different gravel, stone and rock substrate would be more suitable. So the choice of substrate really depends on what types of fish you want to keep.
If you look up the species you're interested in on Seriously Fish (a very reliable resource written by experts and scientists), lots of info about that species requirements are there, from temperature and tank size, to substrate and the conditions of their natural habitat.