why can other nations enter my borders without declaring war? and even after i tell them to leave they get next to my citys and destroy them? do i have to keep a unit on every tile i have or what?

This is one of the things I find very annoying about this game.
Civilization has a warning when units are inside your boarders but in this game you don't know units are there unless you set a unit to sentry or look around the map. You can also be at war and not know about it.
In the early part of the game research shipbuilding so you can stop ships from moving units into territory from sea by placing two three ships next to each other on the coast near the other civilization. This also slows down their territory expansion and works until they research hull making.
On land you can you can place a row of units on every other tile near the other civilizations
border. It is a good idea to try to build a city on any narrow land area between oceans or lakes to make the land border shorter.
Eventually I get pissed off and attack the most annoying civilization.