i recently bought a mic which goes directly to USB which cuts out the options of a mixer. I use it in Garageband and im picking up alot of background "hiss" and im wondering if any of you know how to reduce this on garage band or in recording im thinking of taking it back so do you have any suggestions on a vocal/acoustic mics
There's an option on Audacity that allows you to remove a certain frequency from the mix, which in turns eliminates some hiss, although I've never tried GarageBand. I assume it has that option. . . since Audacity is free.

There are some mixers that are compatible with USB mics as far as I know, although they might run a bit more than regular mixers.

Vocal mics. . . industry standard. Shure SM57. I think. I always get the 57 and 58 mixed up. . .
^ psh, obviously you are mixed up. the SM57 is the industry standard for instrument mics. the SM58 is better for vocals, though it is probably better suited for live applications IMO (as well as other peoples opinions) because it is a dynamic mic. works well for very loud singers and screamers though. for most vocal applications however, a condensor mic will be your best bet. they need phantom power however, so i dont think they come in USB form (i could be wrong). a decent condensor at a good price is the MXL 990. not expensive at all and is supposed to be good, especially for the price.

as for that hiss, you should be able to use a filter to remove some of it. this will cut out/lower frequencies in certain areas, so if you can find a frequency to shut out that will remove the noise and not effect your gutiar tone, that is a good option. i find it works quite well for that hiss much of the time. another option is the noise removal effect in audacity. it does the same type thing as the filter, just automatically detects the frequencies to cut and applies it for you. again, you have to be careful not to get rid of your guitar tone, but you can turn down the ammount of effect. as for garage band, ive never used it so i dont know what it would have to help you, though i would assume it has some sort of filter you could play with.