I use a nikon 5100 and recently I bought a third party speedlight( altura). Since I am a beginner of flash photography I used the TTL MODE With auto setting in camera but my photos are alaways dark and when I turn into manual on both the flash fires but pictures are black too even inhigher iso..ANy suggestion for me
If you want to learn flash imaging, you have an amazing travel and discoveries awaiting; flash lightning can be challenging both at technical and creative point of views. In that case, I recommend to select books, learn theory, practice and so on. It is always very interesting to learn with limited hardware and progress and improve your equipment as you go. Eventually I strongly advise you to acquire a flashmeter (I got the Sekonic cine which I recommend, but it's more than ten times more expensive than your Altura speedlight) and learn with it.
If you want to improve your TTL results, and let's say that your rig is functionnal (it might have been wise to check compatibility before buying, I can't say anything about this), I would make more and more images and learn how your system is reacting to the scene, and eventually choose appropriate settings.
You might want to distinguish flash only images (exposure in on manual, usually with f/5.6-f/11 and sync speed ~1/200), and flash for fill-in situations, which require different kind of reactivity and technicality from the photographer.