I want to trade with a villager but I need to start farming beetroot, all I have is the beetroot itself and no seeds, how can I get them?
You will get 0-3 seeds every time you harvest a fully mature beetroot. Minecart chests also frequently have beetroot seeds in them.
Personally, I would consider recasting the farmer until they offered wheat as a trade. Beets, carrots and potatoes are very inefficient trades.