Criteria to get Scholarship/Assistantship is simple:

(1) Good GPA, above 3.5
(2) Good GRE Scores
(3) Good Reference Letters
(4) What subject you're planning to study (subjects such as Natural Science: Bio, Chemistry, Soil Science provide more Scholarships/Assistantships)

Lastly, you've to be Smart/Intelligent.

I hope this clears your confusion.
Also, Email professors and ask them if they can provide you Scholarship/Assistantship. It helps.

Good Luck

Purana Kagaz.