Here a little guide I made (Sorry about my grammar, i'm french :)

Modding is realy easy, you can mod everything in this game.
You'll loot some cards while playing, they are used to mod weapon and character, or any other thing.

Most of the mods (cards) can go rank 10, and some rank 5.
By upgrading your cards to rank 10, you'll get the most effective cards

For example:
Shield Mod [Redirection]
at rank 10 will gives you +440% Shield Capacity
and at rank 1 it's will gives you +40% Shield Capacity
It's the same thing for the vitality mod called [Vitality]

There's also the corrupted mods
They can be earned by oppening vaults, go do some search on www.Google.com

Making money is simple, you need to do tower missions (they gives 20k credits every 10 rounds or more)...
or do some alerts, or normal missions
but making the premium money is realy long, it's take a lots of times.
First, you need to make premium money, called platinum
it's can be earn by buying them or selling rare primes items (blueprints only) or by selling rank 10 mods or rare mods, or some corrupted mods

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=277976334

First, here is the average price of some items: http://wftrading.net/

You can sell in the trade discution, use this guide, it will help a little:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=277976334

Here are some little world people using for trading:
WTS means: Want to sell
WTB means: Want to buy
LF means: Looking For (mostly used in recruting chat)

You'll need a clan to trade, buy or sell. Or simply, join the dojo of the traders (if they have one)

They can be usefull for crafting exclusive clan items, or going into raids or, the most usefull thing, trading. You can trade by oppening the trading machine.
There also the duel arena, and lots of other thing, like some parkour place (for training).

Finally, the most important thing
Spending your credits and PLATINUM
Credits are used for making Blueprints, or buying them or many other thing.
Platinum is the most usefull thing, you can buy everything with this, its worth a lot.
For example, it's 5$ for only 80 Plats (If I remember well)
But you can earn them by following what I told you before in this guide.

What to buy with plats (platinums) ?
The only thing you should buy with plats are:

Inventory slots
Warframes Slots
Weapons Slots
and, hard thing you can get like:
Vauban (a warframe), he is realy hard to get, but you should wait (his three blueprints parts drop rarely, and they are only obtain by doing there alerts.

You can find alerts notificator everywhere, on phone also
Here a good one for pc (you can allow the sound, so its will tell you if there's a alerts up)
http://deathsnacks.com/wf/

Bonus: You should try to get rhino, he is realy op, but slow
I hope this guide helped you.
You can find many more guide on google or at the warframe hub.