Basically a surfboard has 3 main functions:
Duckdiving(getting under waves)
Paddeling
Riding
For your 1st board I wouldnt worry to much about duckdiving as you will proberbly be in the foam till youve got your popup and riding/balance dialed.+ you dont need to really DD until your tackeling bigger days as you can just paddel over/through smaller waves.
A thicker board will make paddeling alot easier as most of your body weight is above water so you have less drag as you paddel, A good starter thickness would be around 2 1/2 to 3 inches thick.
The width of a board will help with stability with both paddeling and more importantly poping up(getting to your feet) and riding so a board over 20inches would be ideal for a learner as it gives you a little room for error on foot placement when 1st popping up.
THe longer a board the easier popping up will be as its again less critical where you put your feet, stopping alot of pearling.
The other advantage of having along wide(mega floaty) board is that getting into/onto waves will be alot easier so you can be catching anything and everything that moves allowing you to progress a lot quicker!
Most(not all) surfers opt for a minimal as their starter board as it ticks all the above boxes and for your size weight a 7'6 would be an ideal 1st board, but something like a fatboflyer or a huge 7'+ thruster style board would ok but not as easy as a minimal.
Yeah you will need to downsize at some point but as so many people are getting into surfing its real easy to sell your minimal for not much less than you paid for it (i managed to sell my 1st board for £20 less than i paid after about 5 months use

)
The other thing is if you can turn a minmal when it comes time to downsize you will find it soooooooo much easier with turning a smaller board.