Some newbie stuff:
Don't spend your platinum on weapons or warframes unless you absolutely know what you're getting into. Spend your starting platinum on weapon or warframe slots as you want them.
Learn how to mod your weapons. You always want your best "base damage" mod on your weapons. That means Serration for rifles/bows, Point Blank for shotguns, Hornet Strike for secondary weapons, and Pressure Point for your melee. Fusing these mods to be more powerful is a big part of how you "level up" in this game.
You'll start out with Damaged versions of those mods. It's up to you if you want to rank those up with fusion, but I'm sure if you get into the GAF guild and ask nicely, some nice players can hook you up with undamaged versions once you're able to trade stuff. It took me a really long time to get a Hornet Strike mod, but that's luck for you.
Modding for your warframes themselves is generally more flexible than modding weapons, but early on I would focus on Vitality (health) and Redirection (shields) for survivability. You'll also have Damaged versions of these mods, but you should quickly find undamaged ones yourself if you haven't already. Much later on, when you have means to heal yourself, Redirection falls off in value a lot, but Vitality is always great to have.
Rank all your stuff up to 30 before selling it, if you want to sell it, so that you get all of the mastery XP you can get from them. You need mastery XP to raise your Mastery Rank, which unlocks more stuff for you to use. For instance, if you started with a MK1 Braton, you'd want to rank it up to 30 before selling it and making a regular Braton.
Sorry if that's an incomprehensible mess. There's a lot to know with Warframe.
