hi, I have been riding mountain bike for 2 years and I am interested in learning to ride a unicycle, but I have never tried before…
what kind of unicycle is suitable for beginner? what size…etc…? (I am about 6 feet tall)
any advice on getting started?
thanks
Hawo my friend, my name is Harry a.k.a Hazmat 
Let me be the 1st to welcome you to the forums. We’ll help you with whatever questions you may have.
I think a Torker LX would be a good starter, seeming as this was my first unicycle i learned on as well. One thing to do though is change the seat as it will become uncomfortable at some point.
Hoped this helped you my new friend.
Take care, have fun and enjoy
Hazmat
should I choose a 20" to start with?
Yes definitely, a 20" is like the bmx of the unicycling world.
well,
20" good for tricks, freestyle, and riding around
24" good for mountains, faster tha a 20", works just fine for learning
26" same as 24, but a tad bit faster
A cheap 20 is great
You can use it now to teach yourself, and later to help others. Worry about getting a real nice one later when you know more about what you want.
My friends actually prefer learning on my craptex 20 instead of the shiny KH.
They like how I don’t shudder when they smash it ! 
A used 20 for 20 $ would be best. New, there is some garbage out there to avoid. Hazmat’s idea of a cheap Torker sounds good.
If you can, consider investing in a good seat. They might upgrade it for you cheap if you order it with a new uni.
Personally, I’d go for the 24". the 20" might be slightly easier to learn with, but the 24" is more useful to have once you know how to ride.
…others may disagree.
get a 24 for sure
With your height, I’d suggest a 24"…but it’s all preference. Both standard sizes are gonna be equally challenging to learn to just ride on. And LX all the way - don’t make the mistake of getting a CX, like I did.
If you need any help learning to ride, check out the link in my signature.
My first unicycle was a Bedford 24", my brother and I split the cost and shared it. We did fight over it a lot but we helped each other learn so it all worked out. We both bought another one so no more sharing. So if you don’t want to spend too much split it with a friend or sibbling.
I would suggest a 24" because its fairly versatile. After that you can decide specifics. Good Luck 