As you have limited resources it is important that when unlocking skills you choose the right ones to invest your Skill points and money in. You can have two skills active at a time and you should go for the passive ones as they will add up as you unlock them. Consider the prerequisite skills for Frag Grenades to begin with as your active skill and plan your progression accordingly until you can unlock Rocket Man or AM Bomb.
Regardless of what skills you pick the most important thing to consider when working your way up the game’s skills tree is to plan ahead, take a look at the endgame rewards in the Survival, Combat and Agility categories, as well as the hybrid sections because you won’t be able to accumulate enough points to unlock every one of them.