Your main priority should be to progress through the star chart and clear all nodes, while doing that you will continuously unlock new sections of the main story, complete those as well.

This will slowly unlock all the (almost) endless content, game modes, frames and more that warframe has to offer.

Try out new weapons and frames along the way and always try to level something on the side so you keep increasing your mastery level.

Don't make the same mistake I made. When progressing through the story, you'll eventually unlock an operator. I won't say what it is, but when you do unlock your operator don't ignore them, get a decent amp and amp parts and level them up. Quest Lines later will require use of your amp and void powers associated with the operator.

You're going to want to do quests when they become available and the Star Chart is generally more important than the Open Worlds (Dungeons as you call them). They unlock new quests and more of the story, and more stuff is from them than the Open Worlds. Some gameplay tips:

• Weapon mods: Damage categories are multiplicative to each-other, multiple mods within a category are additive. So, you'll do a lot more damage if you put different categories on than stack up any individual one. The categories are Basic(ex Serration), Elemental (ex Cryo Rounds), Multi-Shot (ex Split Chamber), Critical (ex Vital Sense), & Faction (ex Bane of Grineer). Note: Faction damage only works on the specified faction.

• Damage Types: There's several types of health and they each take more or less damage up to a plus or minus 75% based on which damage type you're doing. The common health types are Ferrite Armor(Grineer), Shields(Corpus), Infested Flesh(Infested). To do the most damage use Corrosive Damage against Grineer, Magnetic against Corpus, and Gas against Infested. These are all hybrid elements made with two different elemental mods. Note: This is something of an exception to the above point, making the right element is worth the extra mod but otherwise add a category of damage.

• Weapon Presets: When you view a weapon or warframe you'll see an A, B, and C at the top of the menu. These are your presets, you can set up any piece of equipment three ways and be able to easily change between those three builds without needing to take notes or memorize it and manually make the alterations. This makes making use of the first two points much less of a hassle. Note: You can rename them so you can tell at a glance which preset does what even if you've been away for a while.

• Survival: You have both Shield and Health in most Warframes. The Shield takes 50% damage from all sources except Viral & Toxic which ignore shields entirely (for you and the enemy). Then you have health which takes an amount of damage based on your armor number. Some Warframes primarily use Health while others primarily use Shield to stay alive. Gauss is a 'shield-frame', meaning that he relies on his shield. As such you're going to want to focus on improving his shield capacity and regeneration when you're feeling a little too squishy. Note: Shield frames struggle fighting the Infested because they use Toxic & Viral, Health frames almost universally have an ability that allows them to self-heal and they need high armor to stay alive.

• Agility: Speed is life if you have fast reaction time. The faster you move the more inaccurate the enemies are. I believe this is more than a linear relationship (ie you go n faster they shoot n+x less accurately). Most Warframes can move fast enough to dodge most incoming fire if you've got the reaction time to handle it. Gauss and Volt are the best warframes for taking advantage of this.

• Nightmare Mode: As you unlock more of the Star Chart you'll occasionally find one or several missions you've unlocked highlighted in red with a little swirl icon. When this happens you can choose to do that mission with an added penalty that it will show you. If you complete the mission you will receive a Nightmare Mod each of which has two positive effects. These things are frequently worth the effort and you'll frequently be able to get randoms to help if you run it open to the public. If it's too hard or just annoying ignore them, the mods are nice to have but you could trade for them or just forget about them for now.

I think I'll stop for now as it's past time for me to get to bed and I don't want to make this too big a post.

Saryn is good. Gauss too, but you'll probably have a much easier time making a build that can tackle higher difficulties with her than with Gauss

Edit: though, if you have Gauss prime but not saryn prime, it's more efficient to invest formas and cells in gauss, since you'll end up ditching saryn for her prime version when you eventually get her. Actually I can probably give you a few prime parts for her and help you farm the rest if you really want to play her

If you're interested, I can invite you to my clan and help you progress ingame. The best goal for new players to have is to unlock more planets. The planets are connected with junctions that each have a list of challenges to complete before you unlock the next planet. The challenges teach you how each planet works, teach you important game mechanics in the order you should learn them, and make sure you're keeping up on the main story.

If you're happy in Fortuna, you could also just hang out there and increase your rep with Solaris United, but open worlds are gonna start getting a bit hard compared to regular missions unless you have a great build. If you progress the star chart, you'll be introduced to all the ways of making your gear stronger at a good pace. Plus, the main story starts getting really interesting around Uranus and eventually you'll get far enough in the plot to learn what Warframe is actually about.