Before you read: Please note that we cannot account for every situation or difficulty you might face as a new player. These are basic directions for players who feel extremely lost. You are free to play the game at your own pace and use the equipment you like using.
Step 1.) Choose Volt or Excalibur as your starting Warframe. Complete The Awakening & Vor's Prize quests.
Step 2.) Complete every node on Earth besides Plains of Eidolon, then proceed to the Venus Junction. Hover over the junction Node to see Junction requirements and complete them. Enter the junction and use Excalibur’s Radial Blind to stun the enemy, or Volt’s Shock and Electric Shield to stun the enemy and defend against their attacks.
Step 3.) Ignore Fortuna and Orb Vallis on Venus. Buy blueprints from the Market for weapons, Warframes, Sentinels, etc. as you see fit / can afford, and craft them / level them to increase your Mastery Rank to have a little more fire power and options. Move on to Mercury, and then return to Earth.
Step 4.) Enter Cetus, talk to Konzu, start the lowest level bounty, and enter the Plains of Eidolon through the doors. Completing the bounty objectives is optional, and you can just turn around and go back into Cetus as this will clear the node and open the way to Mars.
Step 5.) Once you unlock Phobos, you can head to the Void to start farming crates for useful mods. (Refer to the command s/essential mods).
Step 6.) If you're struggling with surviving, you can farm the Rhino warframe by repeating Fossa on Venus (Jackal boss) until you have all of his parts. Rhino is a tank and will help you stay alive.
Step 7.) Work on unlocking more planets and try doing every quest available to you.
Step 8.) Once you have finally reached Mastery Rank 4, you will have access to the Hek shotgun's blueprint from the market. It is a powerful early game weapon that can help you if you're struggling to kill enemies.
Still confused on what you should be doing? There is a help and support channel in the Warframe Community Discord server that is full of people willing to help you; feel free to ask any and all questions you may have there.