These talents are almost that important as charm/intimidate. You should develop them as soon as possible so you could open locks and containers and decrypt information hidden in terminals. Without them you couldn't complete or even get certain quests.
Hint! You don't have to improve Shepard's decryption/electronics as long as you take with you character who could handle security at high difficulty level (e.g. Tali).
Like in case of decryption/electronic, you should always have in your squad someone who has first aid at high level so he could cure all its members (this talent is used when you press Y to use med-kit).
You will get an ability to develop this talent at one of the crucial plot moments. It is very important because it grants the unity skill that lets you revive your squad members if they are injured in combat.
Side quests are not required to finish the game, but I suggest to complete at least one of them (126.96.36.199. Rogue VI). It will grant you possibility to choose a subspecialization. You could get this quest after gaining 20 experience level. Talent, which comes with the specialization, will let you generally build up your character.
Hint! Talents described above are useful to all characters, regardless of chosen class or morality. Development of the others depends on your idea for Shepard and on what combat style you prefer.