Farm skill points by leveling up and by chasing the in-game achievements.
You cannot get skill points as drops within a stage though - only by obtaining achievements (1 skill point per) and by gaining levels (think it is 7 skill points per level).
Level up by beating new stages, and by replaying old ones and earning more points than on previous visits. the best way to do that is to start bombing waves a little bit, never more than you can handle, but enough to boost the points you get for beating the waves.
the reset button is usefull if you're trying to get the acheivements at the armor-games level, but don't want to start a brand-new game. best to make a copy of your save, and reset the acheivements on that.