Hi maisatomai,
Here is how you could approach your shooting:
Aperture priority mode:
1. Select the aperture that would satisfy the DOF that you require.
2. Look at the focal length and see if the shutter speed is at least 1/focal length (remember to factor in your crop factor).
3. If the shutter speed is too low, bump up the ISO.
4. If the ISO need is getting too high (which introduce noise), or not able to make the shot, either select a wider aperture or use flash.
Shutter priority mode:
1. Select the shutter speed that would satisfy you requirement. Remember that you might need to use a tripod/monopod if the shutter speed needed is below 1/focal length.
2. Look at the aperture selected by the camera to see if it is adequate.
3. If the aperture is not adequate, adjust the ISO to compensate.
4. If the ISO need is getting too high, or not able to make the shot, either select a slower shutter speed or use flash.