If the front doorknob of your house rattles and feels loose, or just doesn't turn correctly, and tightening the screws on the other side hasn't solved the problem, you should replace it. Today you don't generally replace just a knob, because knobs and door-latch assemblies are sold together as kits, and not all knobs go with all types of latch assemblies. The easier course is to pull out the whole assembly and replace it with a new one. It's easier than it sounds, involving four screws and about 15 minutes of work.
Measure the distance from the centre of your doorknob to the front edge of the door. This distance is the "setback." It will probably be around 6 cm (2 3/8 inches), but some doors vary. Take the setback measurement to your hardware store when you buy the replacement kit to make sure it will fit your setback.
Remove the two long machine screws next to the doorknob on the inside of the door, using your screwdriver. Once they're out, the doorknobs on both sides of the door should slide right out and the latch assembly will be visible within the hole left in the door.
Remove the latch assembly. Working from the front edge of the door, remove the two screws that hold in the latch assembly. Slip your screwdriver into the hole that was left by the doorknobs, and into the latch assembly. Pry it toward the front edge of the door until you can grab it from the front and pull it out. You should now have a knob hole going through the face of the door, and a latch hole connecting to it from the front edge of the door.
Attach your new latch assembly. Slide it into the hole in the front edge of the door in the same position as the old one. Make sure the bevelled side of the latch faces the doorway as you stand with the door open. Attach with the two screws provided, screwing it in from the front edge.
Slide the new exterior doorknob into the knob hole (and through the hole in the latch assembly). Take the interior knob and slide the two long screws provided into their holes, so they stick out from the bottom of the knob's base.
Attach the new door knob. Holding the exterior knob in place with your free hand, guide the interior knob into the hole, turning it so the two screws go into the receiving holes of the exterior knob. Tighten the two screws on the interior knob and test the door.