The 161 is too big for your height and weight, but as far as first time riding goes most boards will seem unstable, however it is probably harder due to the length vs your weight.
Doesnt look like you'll need a wide board based on your shoe size but you didnt say anything about buying boots in your post either. Please please please dont ride in boots bought from a store, I'm talking about work boots or "duck boots", snowboard boots will help alot, probably the most important part in your gear.
Ok it's sorta up to you on the brand here, you'll want a softer flexing board to start on, it is more forgiving and will make turning a bit easier when you get to that point. Most, if not all, board companies have entry level type boards. Since you dont have any gear I'd recommend going to a shop and looking at what they have and talking to the staff there. Make sure when you get boots they fit properly, boots that look good but fit bad aren't going to do you any good, that's why I'm saying go to a shop rather than buying online. However, I went to a local shop (which I hate the people there) tried on boots and then bought them online cheaper once I found my size. Same goes for boards and bindings.
To sum it up, you're looking for an all mountain, soft flexing board, and any shop should be able to guide you to the right board for you. If you can rent a smaller board you may want to try that. I'm not brand-pushing but I have a buddy who has only been riding a couple years,he is on a burton blunt and is totally happy with it (also burt freestyle boots and bindings) but again, the brand is really up to you at this point. Have fun out there!