These guys above me here are telling it straight, great advice! You definitely need to do your research and figure out what "kind" of longbow you want, as the term "longbow" nowadays is pretty danged broad. At its most basic it means a bow where the string does not touch the limbs when braced. That's about it. You can have longbows with highly reflexed, wide limbs and grips that are practically a recurve, or a purist Hill-style that has long narrow deep core limbs and a straight grip, or ANY combination of all those differences/traits. Figure out your desired poundage, figure out how you want your bow to look, figure out what type of grip is comfortable to you, and take it from there. A 68" Hill style and a 58" hybrid with a pistol grip are two different animals.
As a personal recommendation, to get a taste for longbows and a 'middle of the road' experience as far as characteristics, I would look for a 62-64" mild r/d style bow with a low to medium low-wrist locator grip. This should prep you for going to either extreme when concerning longbows- short radical hybrids VS long classic Hill type bows. Some possibilities new or used would be a Bear Montana, Samick Verna, Sky Longhorn or Trophy Pro, Toelke Whip or Super D, Wes Wallace Royal, the various r/d bows by JD Berry, New Wood Vanguard or other models, Martin Savanna, Kustom King Black Forest longbow, and many others.