there are a few posts around about first boards, eg
viewtopic.php?f=39&t=22092&p=165765&hilit=+longboard+stability#p165765(there are plenty of others)
At your height/weight, i would definitely go longer than 7'8 to start. That would be the minimum size I reckon, unless the board was particularly wide and thick. There are plenty of athletic people who realise surfing isnt the same as basketball (or whatever sport they are good at). I'm not picking on you, I was exactly the same.
Aim for a standard longboard of around 9ft or even slightly longer. Be prepared to fall and look uncoordinated for a while, crash and burn and generally not reach your usual standards of sporting skill for a while. But its all fun.
I do suggest a few lessons (a few hours is usually enough).
Oh, there is no doubt that after several months of constant surfing you will probably, at the end, feel quite comfortable on that board and may be considering a new board. Indeed, shorter boards have many benefits.
However, dont get 'conned' into buying that second board first (often a second board is around the low to mid 7ft mark, so you can see that a 7ft8 board is almost a second board). For start, but you can always use longboard forever so its not a waste of money.
Secondly, the 'hump' in surfing is learning how to catch waves and stand up and ride the wave. A longer board will get you over this hump much quicker; a short board will either take longer or potentially you will never get there. So while a longboard might look like something for an old guy or whatever, its by far the best choice for a beginner.
let us know any questions