At the beginning Warframe was not such a playable game, but things changed and developers increased it a lot in the last 2 years or so. The game is now very popular and the most serious guys are playing it. If you never played it before I truly recommend in trying it, I am sure it will get your attention. Before you go though, I want you to read a few things that I think it will help you having a good start in the game.
Here is an old but very good video that it will get you going even if some things are changed in the game since this video the main things are the same:
https://www.youtube.com/watch?v=XCXuLirZq2w
When you first get into game you will have to choose between 3 Warframes. Each Warframe has its own abilities each of them being useful in his own way with its own abilities such as disarming enemies, energy sucking and releasing it to your teammates and even more great features and options from which to choose from.
What I like to do is to stealth behind enemies and kill them. This is very effective because it kills the enemy in in instant no matter his level or its life. This is not hard to do you just have to be patient and wait for the right moment. If the enemy is moving it usually follow a rather simple pattern and it won’t be long until hr is in the same spot again.
Along the map you can there are secret rooms and small areas where you can find stuff such as credits, energy, health, etc. I really like when a game has these kind of secrets in it, it makes it a little bit more playable.
Be careful when engage in a battle, sometimes it is best to avoid battle as you may die and lose all the goods, credits that you got while alive on that map. Don’t get me wrong, you need to fight to farm experience, but if you die you have to restart the game again so be careful in which fights you are engaging on.
If you want to gain more experience in the game I recommend to do first 12 planets without buying anything from the store buying platinum with real money. I don'y afford to buy such things and it’s even more fun to just gain all of the equipment gaining credits while playing a lot.
When you weapon reached level 30 you have to sell that weapon and get a new one which will make the game so much relaxing and easier. The first weapon, no matter with which Warframe it really sucks and you should get it sold as soon as you make it a high level.
The Mastery Rank is one thing that everybody want to get it higher and higher. This is not the case because you will miss all the fun of the game if you are concerned about this. All you have to worry about is to get better gear to level up and to have fun in solo play and in co-op as well.