Look through them!
Different lens lengths are to give different subject magnification; ie how much you fill the frame with something at any given distance.
If you cant get everything in the frame... move backwards or use a wider angle (shorter focal length) lens.
If you want to make your subject bigger in the frame, walk closer, or use a longer focal length lens, (Narrower angle of view)
Depth of Focus changes with camera to subject distance though; shorter, wider angle lenses tend to focus at or near infinity at mush shorter camera to subject distances, so if you were to fill the frame with a person standing, with a wide angle lens, chances are you would be far enough away that you'd be at or near infinity focus and get a large depth of focus in front and behind them. (fore ground, and back-ground would be 'sharp' in the picture)
If you wanted to get a shallower depth of focus, to dissasociate them from the back-ground, (Subject sharp, back-ground and foreground blurry) you would have to get a lot closer to use a near-focus setting that brought the far focus of your DoF closer to your subject.
Alternatively, you'd back up and use a longer focal length lens, that has a much longer near-focus range, so you could fill the frame from further away, and still keep a shallower DoF around your subject....
BUT all subject to aparture settings. Wider appertures give shallow Depth of Focus, small apertures give deep DoF; and your 70-300 has a much wider, hence shallower DoF maximum aperture than your 18-70.
Look through the lenses. Play with the aperture settings, see for yourself how it works, really. Use what you reckon gives best effect.
