Make sure you learn how to bullet jump as this controllable, corkscrew jump can be used as either a defensive maneuver or to reach out-of-range places. To perform a bullet jump start by pressing 'Crouch' (if you are running, you will slide) and then in the middle of this animation, press 'Jump'.
If you aim mid-air while jumping you will slow down time for several seconds, this will give you the opportunity to aim more carefully at enemies.
Upgrade Weapons and Frame Mods:
Make sure you use the majority of your credits upgrading your weapons and frame mods. This includes shield reinforcement, increased cartridge size, armour buffs, and damage buffs. Mods are insertable from one weapon or frame to a new one, and retains all prior memories so don't feel you are going to be stuck with them when you spend your credits.
Buy Additional Warframe Slot:
Use the 50 free platinum you are given when you start the game to buy an additional warframe slot, and four additional weapon slots. Platinum currency is the only way to attain them and inventory real estate is one thing that cannot be looted.