I'm not  sure what to expect from Gen Con. I know there won't be much but I am primarily interested in D&D but I also want to do a lot of shopping for board games, dice, maybe try out minis or painting but I have never been to a con, much less a table top con. How many days should I buy? I was thinking one but I'm not sure how much time I would have.

With all of that you're looking at an extremely busy two days with little downtime and potentially three or four days. That doesn't even account for walking around and joining a pickup game or seeing something cool you've never seen before and want to spend a few generic tickets on to participate in. 

You are probably being misled by the negative chatter you can find here and there; this is absolutely going to be a bigger convention than you can imagine, having never been to one.  My first year was 1997, and the attendance that year was undoubtedly lower than this year's is going to be, and my mind was ABSOLUTELY MELTED by how enormous and packed full of things to see it was.  This is not to say you are guaranteed to want to stay four days, but it is cheaper to do four than two, so... just go for it :)

As others have said, it is going to be bigger than you think.  D&D typically runs a 3 adventure story line at 4 hours each and there are at least 2 different stories.  So there is 24 hours.  

Assuming you want to wander the hall, there are 350+ vendors so far this year.  Assuming you spend an average of 2 min per stall you are looking at over 11+ hours to visit each booth (the hall is only open 8 hours a day so 2 days).  

If you are planning to do 2 or more days, its cheaper to get the 4 day pass.

Now the important things.  Its bigger than you imagine.  Current estimates put the attendance this year at a low 30k people or around half of a normal year.  This is still larger then other game conventions normal attendance.  The ICC is a huge building and its all in use, plus various connected hotels.  It can take 5-10 min to get from one side to the other of the ICC depending on traffic.

There are a couple of different ways to approach the con that I’ve seen.  One is to have tickets to scheduled events from when they open first thing in the morning to when the con shuts down at night.  The other is to leave space between to allow time to discover and try out new things, shop, recharge, eat better food, and the like.  I’ve done both, and each has its charms, so you’ll have to decide what is best for you, based on health, stamina, interest, personality, etc.  I’ll second what folks said above too, there is going to be a mind blowing amount of stuff to do even in a reduced con like this year.  

A few other thoughts:

If there are specific things that you know you want to do, try to get tickets for those up front.  It’s less stressful than wondering whether they will be available later.  That said, for larger events/game hosts, they can often accommodate walk ups with generic tickets, so always give that a shot. Baldman Games runs the D&D Adventurers League games at the Con, and in the past have bent over backwards to seat everyone who shows up.  

Speaking of D&D AL, note that it is organized play that has some specific rules 5e character building etc.  So it is worth checking out that info on the official D&D site ahead of time, so you can make sure your character is ‘legal’ when you show up to the Con.

1 day is not going to be enough to play some D&D and walk the dealer hall.  Get the 4 day badge and get it before Event Registration July 11th.  You'll want to have tickets
to events that appeal to you and it is usually a mad rush on that day.

Seeing as I've been playing D&D a lot at Gencon over the past couple years, I can tell you what to expect for D&D offerings.  BMG does frequent the forums so they can correct me if I am wrong about anything.  It will be hosted in the JW Marriott, which is connected in the northwest corner of the Convention Center.  There will be 11 slots of 4 hour games being run over the 4 days.  Level range of the games this year appears to be Tiers 1, 2, and 3, which is levels 1-16.  Also, if money isn't a problem, they offer a special D&D Experience where their best DMs run groups of the same 6 players in an adventure which is run over 4x4 hour sessions.  Once the Event catalog is released on July 2nd, you'll get a better idea of what is being offered.

NASCRAG is switching back to D&D this year!  Please keep us in mind if you're looking for something fun.

I've found the key to getting the most out of the exhibit hall is to plan your time there, and prioritize the booths you want to visit.  Once they publish the full list, there is a link to each vendor's website with it.  Check out those that look interesting, and then plan on going to those booths.  Also, any games you might find interesting, try to visit those booths first.  As you make your way among your chosen booths, you can get in the "wandering" phase as you pass other booths.

Don't forget to schedule or at least look at some entertainment/film fest events.  They are fun downtime and offer a nice break from games and interacting with people. 

Another gem not to over look is the art show.  Its in the back of the exhibit hall.  Its great opportunity to find some new art plus chat with the people who made it!

Welcome to the Con.  You wont get everything you plan to do, but more than you expect.  But it will be loads of fun!

Bring some hand sanitizer, cough drops, and hand wipes. After three days of talking over crowds the throat gets a little meh. Also a water bottle is handy as there are free refill stations. 

I'm a bit biased but a cool two hour block might be a Freeblades demo with DGS Games. It's a mini skirmish game but it is basically one balanced D&D party vs another trying to accomplish the same objective. Lots of fun and a great company that really loves there players.

Leave yourself some free time between events just to catch your breath a bit. And don't be afraid to just jump in somewhere. Some of my best experiences came from just asking "do you guys have room for another player?"