Slicing bread is one of these deceptively difficult tasks. Any time you see someone slice bread, it's all over in a few strokes, and each beautiful slice is of the same uniform thickness. You try it yourself, and your 'slices' are wedge-shaped, ragged and crumpled. What went wrong?
Firstly, you need a bread knife. It's possible to cut bread with other styles of knives, but without a serrated edge, the bread will get squished. If the knife is not long enough, it won't cut all of the bread, and the edges will be all ragged. Your knife should be at least twice as long as the width of loaf you're cutting. Some carving knives work, if they have the length and serration, but be careful with the sharp, pointed tip. You're going to be cutting all the way to whatever surface the bread is resting on, so use a chopping board to avoid damaging it. If possible, use a non-slip one, as you will be excerting a quite a lot of horizontal force on the knife.
To get a good cut without crushing the bread, instead of using downward force, use back and forth movement of the blade to cut the bread. The serrations of the blade will cut through the fibres of the bread, without compressing them downwards. Cutting through the crusts on the top and bottom takes more pressure than the rest of the slice, but don't get carried away.
To avoid tearing the edges, make sure the tip of the knife always stays outside the loaf. To make good use of the knife, aim to push the knife forwards until there is an inch of blade between the bread and the handle, and to draw the knife backwards until there is an inch of tip protruding from the bread. If the tip enters the loaf, instead of cutting the crust, it will push it outwards, tearing it and leaving a ragged edge.
To get straight slices instead of wedges, pay attention to the angle of the knife. If the blade is held at an angle off vertical, the slice will not come out straight. No matter how much downwards pressure is applied to the knife, it will always make a cut parallel to the blade.
Be gentle, use a good sawing motion, and make sure to hold your blade upright, and you too can impress onlookers with your perfect slices!