if i get 50 marks in jee mains.which type of college i get ( branch cse).

If you will get 50 marks in JEE Main your rank will be more than 2 lakhs.
At this rank there will be less chances to get a seat in CSE branch you can try in private colleges through JEE Main.

50 is very less score you should aim to score good marks in JEE Main. Try to score more than 130 marks in your exam.
For preparation, try to work on your concepts you can take help of NCERT books for that, Go through previous year papers to understand the exam pattern and level of questions. Solve as many as MCQs for your practice and complete mock test series on regular basis to analyze your performance.

All the Best!!