Ok so this is my bands first gig and we need to figure out a good way to mix our music. right now our equiptment consists of 3 Crate GT212's, a Fender Bassman 250 bass combo amp, and a 300 watt Behringer. We have two guitars, a bass, 1 lead vocal mic, and 2 backup vocal mics. right now what we are doing is feeding all the equiptment into the back of the Behringer to mix it, then using all the other amps as extension speakers. the only problem with this is, i cant use the distorshion on my amp because it is acting as an extension cab, so i have to use the distorshion on my metal zone, would i need to buy a seperate head and feed the guitar through that head before mixing it? the other problem we are having is with whistling, also the lead mic wont get loud enouph and when i turn the channel up it hums and feeds back really bad, could this be because of cheap cables? what do we need to do to get a better sound without killing our budget? also we need some sort of monitors... thanks for any help anyone can give!
woah that's crazy.

You first need some monitors so that you can actually use your amps the right way.
The feedback/noise might be caused by cheap mics or a cheap mixer, but in this situation, I've never found cables to be the problem. Cables don't cause feedback, usually.