Well, firstly you're going to need to have additional light like guys above say or at least GOOD amount of sunlight inside of your room. If you want to do it at night, it's going to be hard without lamps (and I don't mean home bulbs and it's yellowish, ugly light).
About a camera, don't use the one in mobile phone (most of them suck and if someone thinks that his is ok, well, he's wrong
).
But let's say you're fucking in the middle of the day and the light is ok.
Then you really need a
tripod (luckily they're cheap), don't try to put your camera on a tv or any furniture, only a tripod can let you set the camera the way it need to be.
Put your partner into bed, set the stage, tripod, camera, aim the camera at her, keep in mind that you've to be recorded too, so don't zoom too much and left some free space around her for yourself.
And I know it's pretty lame, but it'd be great if you will be able to zoom while fucking, it's a big distraction but since you can't afford two separate cameras then search for something with remote controller, but not focus too much on it while having fun.
About the cams, get something with FLASH cards ONLY, don't go for HDD (!). I know HDD looks better, but it takes the hell out of the battery (-20-30% shorter time of recording) and HDDs (even in cameras, excluding SDDs, but you haven't cash for them) are easy to damage, plus they never last long (you will be able to use flash card camera years longer).
Optical (not DIGITAL) image stabilizer is also a very important factor
Optical (not DIGITAL) zoom is the real one, digital is useless
Don't think you will be able to record at the night if the camera has "a lamp", in new models it usually mans one LED bulb
About the model, I'd choose between:
- JVC GZ-HM 330 (AMAZING quality of recordings, remote controller included, a bit TOO small, records only up to 50 minutes on standard battery,),
- CANON HF R17 (a bit lower quality of recordings, but working up to 70 minutes, digital stabilization),
- PANASONIC HDC-SD60 (another great cam, rec quality a little lower than JVC's, but works longer and has better mechanics).
I hope I helped.
It's very hard to find nowadays a recent camera for less than 500$ i dunno why, but companies are probably focused too much on dSLRs and don't give a shit about cam recorders...
Cheers.