Check out this post by the creator of gmat club. This is where I started about 3 months ago before I knew anything about the GMAT. Went from a 570 to 710 after serious studying (150-200 hours). Purchase some books or a course after you have a baseline and go from there. If you're going to do the GMAT though decide where you might like to go and make sure they accept GMAT scores: gmatclub . com/forum/gmat-study-plan-start-your-gmat-journey-80727.html
Once you've decided if you want to go for it or not, I would encourage you to put 110% effort into it. 3 months of prep just a couple times a week will most likely not do it. You have to dive in and take is seriously unless your diagnostic starts out high. Once you've had some experience the GMAT can even be used on your resume for I-Banking... but generally your score has to be over 700 for it to look good. Trust me, no one stumbles onto a 700. Usually requires serious studying and commitment.
Also, note that the GMAT is changing in 6 months. It is generally accepted that the new format of the GMAT will be tougher and require more studying. Many people and GMAT test prep companies are encouraging individuals that are considering the GMAT to take it before the new one comes out to help maximize their score out of 800. The new test will drop one of the essays and add a totally new section with separate grading.
When it comes to what I-Banks like in candidates at your age; they are looking for individuals that showed strong performance in undergrade and have the right mentality. I wouldn't necessarily tell I-Banks that you are considering GMAT b/c they may not want you to leave in a few years, it's hard to say b/c each will look at it differently. Better to be safe and not tell them plans, or put GMAT on resume. Bottom line is though, you really need some work experience to be considered for a top MBA or Masters in Finance. So best of luck in your job search, and if you have extra time and know you want to go to school in the next 5 years, definitely use your spare time to study for GMAT. It is extremely challenging to study while working full time. Best of luck to you!
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