This relies on dice rolls, things like positioning, height, and similar are factored into rolls as bonuses and negatives. Getting flanked removes your cover bonus, having the high ground gives you a bonus to hit.
Movement starts at a base 40(feet, meters, squares, whatever you want to issue for measurement) and certain pieces of equipment like machine guns reduce movement speed. A machine gunner from the example above can move at 30 units rather than forty.
AND movement is two step, you can move 20 units and attack or do whatever action you want, move over 20 units and that's the turn. Specific movement speeds can be altered but a general rule is that half the standard movement speed is a move action and over that is a full turn action.