For those just starting MAG, the temptation to use Skill Points to upgun as fast as possible is very understandable, but should be avoided as it will slow you down in the long run. In order to get as many points as possible and be an asset to the team, the first two purchases (at 2 points each) should be the medkit and the resuscitate skill in that order. Judicious usage of the medkit on your team-mates can net you a huge points bonus in each match, boosting your progress through the first ten or twelve levels.
After gaining the above skills, opinion differs on what skills to get next and the optimal skill path depends greatly on preferred playing style. Although both Improved Shot Detection and Sprint Recovery are highly recommended, going for a better weapon or upgrades to the starting weapon is often just as good, especially from a short-term viewpoint. Once you have determined your preferred play style, it may be a good idea to concentrate on getting all the skills (such as rapid reloading or steady aim) in the associated skill branch, since it will lead to a boost in accuracy and a reduction in vulnerability caused by reloading. This will allow you to kill more enemies, thus gaining more points.
The more advanced skills are highly specialised and should be used only by those who know what style of play they wish to use. For those who like to infiltrate enemy lines and hit them from behind, Stealth is a must-have. For those who routinely find themselves setting off claymores, explosive resistance is a good thing to aim for. Since gaining these skills costs points which could otherwise go into getting bigger guns or better grenades etc., it is highly recommended that players think hard about the tactical advantages and disadvantages these options present with regards to their preferred style of combat.
Various combat roles can be significantly enhanced by choosing various combination of skills. Those who prefer close-in front-line combat often find that chemical and/or explosive resistance and boosted health can give them an unexpected edge in charging prepared defences or fighting off hostile forces. Medics will find the Improved Resuscitate makes them far more popular with their team-mates, while snipers will appreciate the rear security offered by well-placed Claymores while they are looking through the scope for their next target.