I don’t recommend you use Genki as a beginner. In the long run, that book won’t give you a solid foundation in Japanese grammar. It’s not bad for learning basic conversation, however it’s not all that comprehensive. There are better books out there.
I personally recommend “Tae Kim’s Guide To Japanese Grammar”. From start to finish, it’ll provide you everything you need to get on your feet. Best part it’s online, simple, and completely free (http://www.guidetojapanese.org/learn/). Though if you want you can buy the physical book (completely same as online version).
After you finish this guide, I recommend the book “A Dictionary of Japanese Particles” by Sue A. Kawashima.
Hmm... I don't want to recommend you a "text book", because I'm studying English though, I just finish reading "The List (by Siobhan Vivian)". It's the book for teen though, pretty easy and fun to read for me learning English. Hold on...Let me think...