[G&K] i always gets invaded

i am not the best civ player i have started to play on prince.
i always forget to build up an army before its to late.
i want to build a lot of building and wonders in my cities so i do not have time for building armies in early game.
i can defend the cities but its allways end with extreme unhappiness and bankruptcy.
any tips on balancing unit making and building making?