Any warframe can get you through any star chart mission, just learn how to play them, look for a good build.
Rhino is "easy to get & easy to play warframe", it will help you with any tough situation.
At this point try to look what prime warframes you can get, look for videos, see what you like, ask people in clan.
Gram, Silva & Aegis, Ignis, Tonkor are solid weapons. Karak has very low crit chance and his main damage is Impact (let's just say it's not very useful right now).
What else can you get for mr6:
primary: automatic rifles - Soma (high crit, high fire rate); semiautomatic rifles - Sybaris (high crit, high damage); shotguns - Hek (high damage, ok crit and status chance).
secondary: Atomos - great weapon, good AoE, good damage; Lex - semi auto, high damage, high crit; Twin Gremlins - automatic, good damage, fire rate, status and crit.
melee: Dual Ichor, Tipedo, Dex Dakra.
- modding weapons
Right now we have two main options for elements: Corrosive (Electricity+Toxin) for the armored enemies and Viral (Cold+Toxin) for the most of the rest.
So most of your ranged builds with good crit stat will look like: damage, multishot, crit chance, crit damage, elements. Look on wiki where can you get different mods, look for some dual stats mods like Malignant Force, also you can trade for some of them.
Most of the melee builds: damage, range, speed, crit chance, crit damage, elements. For an "Endgame" melee build you will need mods like: Blood Rush, Condition Overload, Drifting Contact, Primed mods. You shouldn't go for this mods right away, but you will need them at some point.
All of that is just basics and some of that is only my opinion. Ask people, watch videos, read wiki, test everything yourself. Grind a lot. And Always Have Fun.