If the frog says, "build roads first", then by all means do build roads first.
It is really that easy.

And by the way - just play ahead. This game isn`t so difficult. It´s a game, not higher mathematiks. Go ahead, if you make some mistakes it`s fine, you will still win.
Your progress in the game will be quite slow if you ask here before every building you constuct.
If you don`t like the aproach of try and error, you have your advisors and frogs guide - seriously, no one can ask for more.