Mohsin, the way I see it, depending on your skills you have two options:
1) If you can read write in a sylheti flavor then all you have to do is become conversant in non-Sylheti, mainstream Bangla dialect. Antora and Orphy have good ideas on that front. At the same time might I suggest that it is time you got familiar with our awesome body of literature. Syed Mujtaba Ali, Humayun Ahmed, Rabindranath, Nazrul, Jasimuddin to name a few - you'll find that there's quite a lot to read out there.
2) If you cannot read or write Bangla (in any form)then yeah, you need to get some basic Bangla books. My kid's learning Bangla (he's 3) and we've got this books where he can trace the letters and erase them. I'm guessing you'll graduate from that to spelling in a few hours and from spelling to 3rd of 4rth grade level reading in a week. After that go back to option 1.