Just start by shooting enemies in the face. When you feel ready just have a look at one of the system and learn to use it to improve your ability to shoot enemies in the face. Repeat until you understand all the systems.
That's what I've been doing for the past 10 years and it's been working rather well!
The in-game codex will likely give you an idea of the features available to you. Alternatively, you might want to have a look at your profile and view the weapons and warframes that interest you that you have yet to unlock/master. From there you may have a look at the wiki to view how to obtain them. Some such warframes, such as Hildrin, will require you to interact with some extra systems, such as unlocking the ability to equip an archwing weapon in regular missions.