Your focusing technique is correct. As far as exposure goes, I don't know if this will help you understand exposure, but this is one way to try to understand it:
Think of proper exposure (as read on a handheld or camera meter) as a glass full of water. You can chose to fill the glass slowly with the tap open just a little (small aperture, longer shutter speed) or fill the glass quickly with the tap fully open (large aperture, faster shutter speed). So as you change the aperture, the shutter speed needs to move in the opposite direction and vice-versa. Another factor issue film speed. A higher ISO needs less light to make a proper exposure, allowing for faster shutter speeds and smaller apertures. The pictures will be sharper, but higher ISO films will have more grain, showing less detail.
Generally, you will want to chose an aperture that gives you enough depth of field (that is, how much of the image is sharp, measuring from in front and beyond the subject you're focused on (you can consider hyperlocal distance and other niceties later) and still be able to use a shutter speed that will either stop the action in the scene (children running, vehicles moving) and not be affected by camera movement.
Additionally, wider lenses (in the case of 35mm, a focal length of less than 35mm) will have more depth of field for the same given aperture than longer lenses (say, more than an 85mm lens). Also, longer lenses, because they are heavier and record a smaller area of a scene, will show camera movement more than wider ones--so you want to use a faster shutter speed. Most people are able to hold a camera steadily enough at a shutter speed that corresponds approximately to the focal length of the lens being used. So a 28mm lens will not be affected by camera movement at shutter speeds of 1/30th and faster, but a 135mm lens will require you to shoot at 1/125 or faster to eliminate camera shake. Using a good tripod will eliminate that issue entirely.
I know it can be confusing--keep at it. It is very rewarding when you control everything on the camera. You have truly made the picture, not just taken it.