From which subject one should start preparing for GATE and for how much time a student should study on daily basis to get a good rank < 100 ?

Start with the subject you are most confident at. Keep daily goals and tasks for each day and be sure to finish them orderly. Organize the tasks in such a way that you remain active throughout. If sticking to the same subject for long gets boring or tiring, switch. 
Preparation time for GATE depends on many factors, such as individual's aptitude, attitude, fundamentals, concentration level, etc
Generally, rigorous preparation for four to six months is considered good, but it may vary from student to student
Candidate should refer to the syllabus and analyse the exam pattern using previous years' papers and start preparing accordingly
It is important to know what to study, but it is more important to know what not to study
A clear understanding of the underlying concepts is a must since the questions asked in GATE exam will test the level and depth of understanding of concepts.