How would i power 2 6x9 speakers and 1 subwoofer? (and general help please!)?

i already have one rockford fostgate 100w 4 ohm sub set up with a kenwood mono 2 channel amp, works perfect, but i want to increase my sound system and want to add a couple of six by nines, how would i power everything up? buy another amp? do i need a crossover? what even is a crossover? confused! also, how could i power two tweeters? thanks!