for hunting bird what kinds of gear should choise at first?

What kind of bird? Around here bird hunting usually refers to upland hunting such as grouse. Ornage hat and vest, and a shotgun. I have always used 12 ga with a modified choke. But 20 would just as good.

If you are talking ducks, alot more. Decoys, calls.

Welcome and please tell us what birds you have in Bangladesh if this is a legitimate post!

yeah I think you need to clarify what species of BIRD, your after/

But if upland birds, I suggest a GOOD boots, a shotgun your quick and accurate with, a good vest, and best thing to have for about any bird hunt, a GOOD bird dog! just the joy's own owning and having will benefit your entire life!, get you more birds, and a pure joy to watch and par take in a hunting behind your own bird dog!

if, waterfowl if the game here, that will maybe come down to how you hunt them, if from shore, again, good shotgun that can handle steel shot(also needed for upland game pending where you hunt them at) good chest waders, calls/ and some decoys
and again, a GOOD bird dog, for recovering them, and again the joys of hunting and owning one!

if you hunt from a boat? then of course a solid boat that will be stable to hunt out of haul all gear and offer you cover!