Welcome to the bike squad! I have some bad news for you: if you NEVER bike anywhere, your bike seat is going to hurt a bit at first. No matter how comfy your seat, your bum will have to get used to sitting on it. Heck, I've ridden my bike everywhere for as long as I can remember but my bum hurt as hell when I got back on my bike after spending a year in Canada without riding due to not owning a bike over there. However, investing in a padded seat does make a huge difference. So: get a low-cost padded seat and suck it up.
As for bike advice: go to a good bike shop to get advice on the size your bike should have, and to check out what's available. Pump them for all the information they can give you. Thank them effusively, turn around and buy a second hand bike on Craigslist based on what you now know. I suggest going for the simplest, sturdiest, most no frills kind of bike that's still usable for your commute. This will save you a lot in maintenance - moving parts break, need lubrication etc. Example: if all you ride is flat tarmac, you don't need more than a 3-gear bike. You don't need a mountain bike if you never ride in any actual mountains. When buying a second hand bike, be on the lookout for hidden rust - a lemon in bike terms is a bike that rusts through it's axles shortly after you buy it.
Also: learn how to mend and replace a tire and perform the basic maintenance (greasing the chain etc) your bike requires. there's lots of video tutorials on YouTube. You don't need lots of extra gear, for starters lights + extra inner tube + puncture repair kit + pump is fine.
