Reading up a bit about general PA is a good idea, especially as some mistakes can blow stuff up which may not be covered under warranty. Like Jamie said positioning is important.
Put the stacks at least 3 feet forward of the mic stands, so they are in the mic's dead zone.
For a basic desk into amp into speaker setup turn on all gear before the amp. Run the amp volume control at nigh on flat out, put the desks master fader at 0dB and get the mix and overall volume from the individual channel faders. Start with all faders down except master and then get your gain levels using the metering.
Start with one mic and solo the channel, set the gain so it's well above the noise floor but away from clipping the channel. Bear in mind a guy saying one, two etc isn't half the volume he is when he is singing. Then move onto the EQ, start by cutting everything you don't need, i.e for vocals lose everything below about 100Hz and everything above about 16Khz. If you get a problem that needs EQ'ing to solve then try and do it by cutting frequencies rather than boosting them. Any EQ change will obviously affect the gain so you need to keep looking at the meters \ bars to check you still have plenty of headroom and aren't going to clip anything or sink into the noise floor.
You don't actually normally get feedback from the FOH speakers, it's normally the wedge monitors that can give grieve. Best thing for them is a 31 band EQ, again lose everything you don't want and then cut the offending frequencies that are causing the ringing, you soon learn to identify them.
If your running the effects via a normal aux send and returning them into a spare channel then you shouldn't get any problems in general. Just remember the effects send needs to be post fader and the monitor sends need to be pre fader
My band practices in a very small room (probably mot much bigger than the size of a medium-large living room). We find that if you point the speakers towards the wall of the room, away from mics/other amps etc you can push the volume up a bit more without feedback. It's not ideal, but seems to work for a small room.
Cranking up the gain is probably the reason your PA feeds back (or it'll certainly be contributing). High gain means that you're making the mic more sensitive and 'letting in' more sound, for want of a better way of doing it. This means that the input signal is very high, and so even low output settings on your PA/amp will result in feedback. Plus, the mic will pick up other sounds in the room, even if the mic isn't pointing at them.
Rule of thumb is to keep the gain as low as it needs to be to pick up a signal (your PA should have a meter that you can use to guage this, often referred to as a pre-fade listen or PFL), then turn up the output volume. This way, you're making the amp do the work in the output stage, not the mic in the input stage.
If you're pushing the output/volume of your PA up high yet you still need the gain up high to get a decent volume, then either your PA isnt loud enough for what you need, something is faulty, or everything else in the room is too bl00dy loud..! [turn down..!]
In terms of the first possibility, this depend on the output rating of the PA and at what impedance (measured in ohms), and the speakers being used with it. Post some details about your gear and I'm sure someone here could tell you whether you gear is suitable for the job. Check that first, then if it should still be louder than it is (even when used on its own without any other instruments playing), you could have issues. Try to use it with a different set of speakers first if you can, and likewise your speakers with a different PA to try to isolate which might be causing the problem. In my younger days I blew many a speaker horn by pushing the gain too high and making the speakers clip: high input gain = increased risk of clipping/non-musical distortion, which speakers do not like. If you blow your speaker tweeters, you'll get a muddy, quieter sound that you'll inevitably try to solve by increasing volume/gain and forcing the eq. Not a good move...!
In terms of room positioning, just make sure that the end of the mic that you sing into isn't facing or in front of any other sound source, esp the PA speakers themselves. Either stand behind the speakers, or stand at the opposite end of the room and face them. Make sure that there are no guitar amps turned up to 11 blazing away behind the singer either, as this will increase the input signal (esp if you've got the gain on the PA high).
If all else fails, take the PA to a pro to check it out. Depending on what they're like, some will test it out for free and if there's no fault with it they wont charge you (plus they'll have other, working gear to test it with).
It's a Behringer 2442fx mixer into 2 Mackie SRM450's. They're 400w each so I'd expect it to easily handle a band situation.
Volumes are only 50% up on the speakers though so there's room for movement there.
My main concern is that when the gain is almost full and the singer sings the PFL/meter thingy is only up about 3 bars from a possible 12/15. It doesn't even go up to 0dB when she sings. Surley it should be higher than that?
I'm either missing something totally obvious or something's broken.