Using a skill is allays better than relaying on your default ability bonus as they add their associated ability bonus and if used for their purpose may give large bonuses.
For example, shooting a Desert Eagle will require you roll an attack with your Dexterity bonus, but with a — 4 because you are not skilled in it. Because your DEX is 7 your Dex is 2 and your attack roll would be: 1d20+2 — 4
However, if you have the skill ranged combat with 1 level in it you would instead roll an attack with your ranged combat bonus (which includes your dexterity bonus) your attack bonus is now 1d20+3.
Alternatively, if you had used a more generic skill such as combat you would incur a — 2 penalty, whereas more specialized skills such as firearms would yield a +2 and Desert Eagle a +4.
Skill Known, Primary and Secondary Skills
Each character begins play with knowing four primary skills, and may know an additional secondary skill equal to their intelligence bonus. You may know a skill without any points in it it just means you use the — 2,0,+2 or +4 inherent bonus.
At first level and periodically thereafter characters gain 4 skill points + Int point with which to allocate to their skills. Primary skills require one point to increase to the next level, secondary skills require two points to increase to the next level. Lastly the level cannot be higher than its perspective ability score.
For example from our previous example, you character cannot have more than 7 levels on his ranged combat skill because range combat is DEX dependent and his DEX is 7.