There are three ways for you to improve your character's skill levels.
Firstly, any time an attribute changes (either up or down), any skill which has that attribute as either primary or secondary is affected, and may need to be recalculated.
Secondly, your character may receive training from another character (run by another player or the GM). For details on this method, refer to the description of the Training skill.
Lastly, skill levels are increased through practice. Every time your character uses a skill, there is a chance that he will learn something new that he didn't know before. If your die roll exactly matches your character's effective skill level (see the section on Skill Checks in the Introduction), the skill level is increased by one point.
Your character has a skill level of 68 in a certain skill. The GM assesses a modifier of -10. The effective skill level is 58. You luck out and roll a 58. Your character's skill level in that skill increases by one point, to 69.
Since there are separate ways of improving skill levels, you should keep track of how many points have been earned in each way separately. There should be three columns for this on your character sheet. The first should show how many points are from the attributes the skill is based on. The second should show how many points have been earned through practice and training (since training is really just intensive practice). The third column should show the sum of the first two columns (the actual skill level), and is present solely for convenience (so you don't have to keep adding the other two up). If you don't keep track of these numbers separately, it will be very difficult to tell whether a skill level should increase when an attribute improves.