stablish an economy. If you're not trading in dollars, Euros, or other currency, you will need to create your own financial system. Will you base your nation's wealth on gold, on securities, or on a whim and a prayer? While your word may count among your friends, for national debt, you will need some serious collateral for that to be of any use. If you stick to established currencies, you will still need to determine how to fund your government, and the best way to do this may be anathema to the very reason you start your own country: taxes. Through taxation, your government will be able to provide essential services such as a power grid, water lines, a necessary bureaucracy (as minimal as you like), and an army.
It's a fundamental obligation for every state (small or large) to be able to defend its citizens from enemies. Whether this is a standing army, national guard, compulsory service, or some other defensive solution, this will be something to consider when creating your constitution.