what would me my rank if i scored 125 marks in jee mains 2017 . i belong to obc category.

NTA Scores are given in percentile.
Percentile is calculated by the ratio of the number of values to the total number of values . You cant say/depict your actual score/rank from your percentile neither can you calculate it by only marks. To calculate the actual score/rank/percentile you would need to know the number of students who appeared in the examination and the highest score, average cut offs and the highest score. while as marks are calculated based on the right questions you have attempted, out of the total marks you have obtained for the right number of questions marks from wrong questions marked are deducted from the total.

NTA announces the result of JEE Main/Advance in the form of a percentile score in every attempt or exam of JEE Main/Advance. The All India Rank (AIR) of candidates in JEE Main is calculated by taking the best score out of four attempts and are provided after the result of fourth exam. Candidates are able to check the detailed marks vs ranks of JEE Main/Advance after the conclusion of the exam.

Two types of ranks are given at the end:

1. AIR: All India Rank
2. CR: Category Rank

AIR is the overall rank obtained by the candidate respective of their category and CR is the rank candidates got in their respective category (General/EWS/SC/ST).

It depends upon the JEE Main and JEE Advance cut offs, If You manage to qualify the exam by achieving the cut off, You can apply for JOSAA Counselling. It should be noted the cut off for EWS/SC/ST/FEMALES/OBC category is lower than the general category
However It is usually advised to fill the choices as per the preference of colleges and departments, for example CSE in NIT Trichy is your priority followed by CSE in NIT Warangal, you can fill CSE at NIT Trichy at serial number 1 and CSE at NIT Warangal at serial number 2 and so on and so forth. However to get any NIT, you must have qualified JEE Mains and to get any IIT, you must have qualifies JEE Advanced

But the previous cut offs of colleges should be taken in consideration, it is better to research over the cut off of colleges then make a preference list of colleges and fill accordingly

For the college preferences and the courses provided by these colleges and to know which colleges participate in the JOSAA counselling, please go to the below mentioned link: https://engineering.careers360.com/articles/josaa-participating-institutes/amp

For college prediction as per your marks and rank, please go through the below mentioned link:

College Predictor 2019 - Predict Colleges for Engineering, Medical, Law and More

College Predictors – Know your College Admission Chances Based on your Rank/Percentile, Home State, Caste, Gender, Exam Result & Get your Pe ...

Rest for information on any particular college, you can dig their official site for better information