I’m thinking about getting a unicycle. Do you recommend a certain size or type of unicycle for someone who has never been on one? Any other bits of advice? 
Cheaper is better. There are good deals on CL. After you learn to ride, then treat yourself to something nicer. 20" unicycles are safer since you have less far to fall. Wear safety gear. Wrist guards are my #1 item. You’re going to fall, and we want you to get back on the unicycle after that happens. Mostly you will just land on your feet (and for some reason, bystanders think it’s really dangerous when your unicycle goes bouncing away from you). Be sure there’s enough seat post to accommodate riding with a slightly bent knee while placing the corresponding foot on the pedal in the bottom position (this is important to think about when buying). Have easy access to all the tools necessary to tighten the unicycle. You do not want a pedal to get loose, destroying your crank.
There is a lot of good advice and encouragement to be found on this forum. The search function is your friend. Please, occasionally share your progress with the members of this forum. If you feel like quitting, post that, and we’ll do our best to talk you out of it.
Welcome to the forum.
I an into my 3rd month of learning and started on a cheap 18" Uni and have just got a new 26" and I would have struggled to learn on anything bigger than a 20" get a cheap second hand because it will get bashed.
I have only been riding since July 2016, and I learned on a cheap (literally $10.00 Australian) 20 inch unicycle. I have ridden 16", 24" and 29" unicycles since learning and I am so glad I started on a 20". It is a great size for learning on as previously stated above. Easier to turn, easier to mount… the list goes on. I wish you all the very best. No doubt once you start you won’t be able to stop. The challenge and the enjoyment of riding just make you want to continue practicing. I know that I experienced almost 100% frustration initially for the first number of hours of learning, but once you start to make progress it is addictive. You have found a site (here) with great advice and where those with a wealth of knowledge have posted tutorials and the like to assist you with your progress. Persevere! Enjoy! And post some videos of your progress! =)
20-inch is the size that most people learn on.
16 might be too small, and more sensitive / wobbly compared to a 20"
24-26 is further to fall, and can be a struggle to turn and get moving until you get used to them.
A cheap 20" Chinese made Uni will do the job - but you may find that you need lots of replacement parts to upgrade it into a half decent Uni, therefore a decent second hand Uni would have been a cheaper purchase in the first place.
See what is available second hand.
I bought a £16 20-inch from Ebay (then spend another £30+ on upgrades)
And also found a £10 second hand 16-inch Club Trials on Ebay which was an absolute bargain.
Make sure you like the sport / hobby before investing serious money on a new piece of equipment.
I really like riding my bicycle and one day, I saw a man on a unicycle and I thought it was the coolest thing. Would it be best for me to practice on grass? I really don’t want to hurt myself! I was also considering buying a skater helmet.
There is a ton of advice on here I started in a hallway inside for about a week then went outside and launched of a wall lots of crashes your bike helmet should be fine don’t give up .
I wouldn’t recommend learning to ride on grass. It can be bumpy, and it’s hard to get moving, not to mention turning to keep your balance. A skater helmet is a good idea though. They protect more of the back of your head, which is better for unicycling, especially if safety is a high priority.
+1 on that. There’s no easy way to learn, just have to do it until it works!
Would a Club 20" unicycle work for a 5’7" 175 lb person?
Yes, that should be more than adequate.
Yes - I am 175lb, 5’10"
The 20" freestyle will be manoeuvrable and have a fast response.
The 20" club trails has a wide rim, fat tyre, and a stable base to learn from.
The 24" freestyle will be a risk, and make learning a little bit more difficult than it needs to be.
The 38mm wide rim on the 24" club Muni will accept fatter tyres (up to 2.6) easier than the 30mm wide freestyle rim, and have more forgiving 150mm cranks for better control.
In this picture, the 16" trials is at the front / Chinese 20" second / Orange Club 24 freestyle third.
My vote would be to go for a club trials 20" so you can still play about on it as a practice Uni when you outgrow the 20" wheel size.
But everyone is different, with different priorities.
If your second hand uni is cheap enough- you can not lose.
If you buy new - you need to second guess what you might need in 6-12 months time.
I learnt on a 20" about twenty years ago, recently took it up again with a 24" Municycle, very hard to re-learn, I wish I had stayed with a 20" to be fair, but I have now got the better of the 24". Do what all the experts say, a cheap 20" to see how you go first, then, buy dozens of them as the skill takes a hold of you… !! 
Out of my 4x Uni’s, I am using the 20" the most for free mount practising, and riding around the back yard.
You won’t go wrong with the Club 20-inch as a small light weight “grab-and-go” if you upgrade to a larger Uni later.
Most people learn on a 20", and that is partly because most people recommend what worked for them - but what worked for them was what other people recommended, and so ad infinitum.
A 20 feels lower and safer. In fact, the seat of a 24 is only 2 inches (50mm) further off the ground. A 24 is slightly less twitchy “fore and aft” but a little bit harder to steer.
My sons learned on a KH 24 Muni and a KH29 Muni. People have learned on a 36.
There are plenty of cheap 20s around sold for beginners or clubs. Even local bike shops may sell them. However, there is a huge range in terms of quality. You need a decent seat and cranks that are a sensible length.
Best option is to go to a trusted supplier like unicycle.com, browse their catalogue and decide.
Depending what riding you want to do later, a 20 may be easiest (“tricks” and “skills”) or a bigger wheel may be better for distance, speed and varied terrain. All other things being equal, a 24 is 20% faster than a 20. It’s the difference between labouring along or trundling along.
If you take to it, you will soon upgrade anyway!
There is that, but looking back and knowing what I do now I’m still happy with my choice of a very cheap 20 - a 26 seemed a long way up and I’m sure I’d have found it very intimidating to learn on, and that’s only 1" higher than a 24! A 20 is still my weapon of choice to learn new things, even if the aim is to transfer the skills to a bigger wheel, and I still wouldn’t prefer a 24 for that.