how to secure my network!?

Hi all,

i recently bought a cisco router 2811 and made it my LAN internet Gateway.
everything is working perfectly, i configured secondary ips, did shaping, configured access lists and so on!
but theres one prob thats haunting me!
network security..
the fact remains that anyone which has access to my network (wall sockets) could plug in his laptop and simply acquire a dynamic ip! and hes got full access to yy network!

is there a way that i could configure my router to deny new pcs unless if i allow them to acquire an ip?!

any help would be appreciated 🙂