So, this is my advice based in my experience:
You must be already familiarized with DAX, Power Query, T-SQL and SSAS. Doesn’t mean you cannot learn it and pass the exam but without some previous knowledge about these topics it will surely take longer, and you are increasing the probabilities of be unsuccessful.
If you are only starting now working with Power BI, better to consolidate the basics before taking the exam. There are plenty of invaluable resources in the web where you can learn, for free! From Microsoft Documentation to gurus’ blogs such as Curbal, PowerPivot Pro, RADACAD, SQLBI, Guy in a Cube… and many, many others.
In my personal case, I already had a solid 2+ years learning Power BI in a work environment when I decided to sit for the exams. It doesn’t mean you can’t do it without such length of experience, but I think it is important to notice it.
The first book I read was Power Pivot and Power BI – by Rob Collie & Avi Singh and I honestly recommend it.
Then I did the Microsoft courses Analyzing and Visualizing Data with Power BI and Analyzing and Visualizing Data with Excel in EDX which are also the online training recommended by Microsoft in the Exams page.
Note you can just Audit the curses, i.e., you don’t need to pay for them unless you are pursuing the certification. In my case, yes, I did paid but was because these were part of the Microsoft Professional Certification in Data Science I was taking at the time.
When I decided sitting for the exams (Summer ’18), I bought the official exam reference books Exam Ref 70-778 Analyzing and Visualizing Data by Using Microsoft Power BI by Daniil Maslyuk and Exam Ref 70-779 Analyzing and Visualizing Data by Using Microsoft Excel by Chris Sorensen.
The two Exam Ref are well organized, and one can see Authors have put a lot of effort to create a study path. Chapters follow the Exam structure and reference many extra resources. Kudos to them. Important, I studied for one exam at the time, 70-778 first and then 70-779. There are so many similarities between Power BI and Excel that can trick you that I won’t recommend to study for both exams at same time.
A third important resource I used was the white paper recently published at the time: Planning a Power BI Enterprise Deployment by Melissa Coates and Chris Webb.
This paper has been extremely useful for me until today and still one of my main references to a complete view of Power BI Environment.
As Practice Questions, I summarized and reviewed the questions of the Microsoft Curses, the Exam Ref books and I did the free test provided by Accelerated Ideas.
Following the practice examples in the Exam Ref books is also recommended and a good way to train.
The exams length time is between 120 and 150 minutes but you must read carefully the case studies and the questions, hence is important to manage tightly the time. There are questions that all answers are right, and you most select the most complete. Be aware of these.
And finally, be calm! If you get stuck move for the next question. You can come back to it later.
I hope it would help you and good luck for the exams!