hi,

see apart from nits and iits all government and private colleges takes admission for lateral entry student in the second year of btech. you may appear in the lateral entry exam conducts by the respective state exam commission to become eligible for admission.

now coming to gate preparation the major subjects which are in gate syllabus starts from second year. however first of all you must have clear concept of the any subject you want to start preparation. however I would suggest once you have cleared the university syllabus then you should go for practicing for gate exam. the ideal time for starting gate practice is from third year.

after btech you have several options you may go for job as per your stream or may go for masters course clearing gate or individual entrance exams conducts by different university . you may go for mtech or mba or integrated phd course after completing bachelors.