Best way to protect my Network

Hello,
i am having a small office where by i want to set a network security on all the computer they are like 25 desktops, and 10 laptops, i already set up a connection on this office all the computers are connected , thru cable connection straight from cisco switch, and the laptops also are connected thru my wifi , so all i wanted to ask is , how can i monitor all these computers , using my computer server? , so that no computer outside my network should get an access thru my wifi or my network, unless i give them access myself

my question is , what is a way of protecting my network from the intruders? so that i can i make sure .. no one gets an access to my network unless i give it to him ...



Your suggestiosn and help would be much appreciated ...