I want to start making a MMORPG so I have been doing some research but I still have a few questions.
1) Is DirectX multi platforming?
2) Can I render 3d models with OpenGL?
3) What do you think is better OpenGL or DirectX?

Other than those questions I want to write the game in C++ and use SDL_net for my Client/Server... do you think that's a good idea or should I use something else? If so please let me know what would be best. Thanks in advance and thanks for taking the time to help me and read this thread.

I want to start making a MMORPG so I have been doing some research but I still have a few questions.
1) Is DirectX multi platforming?
2) Can I render 3d models with OpenGL?
3) What do you think is better OpenGL or DirectX?

Other than those questions I want to write the game in C++ and use SDL_net for my Client/Server... do you think that's a good idea or should I use something else? If so please let me know what would be best. Thanks in advance and thanks for taking the time to help me and read this thread.

No offense intended, but those questions suggest you really need to start with something much smaller before trying an MMO game. The direct answers are:

1) No, not unless you run through an emulation layer such as Wine or related.
2) OpenGL can do basically everything that Direct3D can do, it is just a different API for the most part.
3) Direct3D has actually become a better API than OpenGL in some ways (not trying to start a flame war, "some" parts are cleaner as of DX11) . But, DX is not cross platform and OpenGL is what you will use most of the time on other platforms.

Having said that, C++ and SDL are fine, I would highly suggest doing something very simple and getting used to things first. I've been making games as a hobby and now professionally for over 20 years, I still don't know all the ins and outs to create a decent MMO by myself or even as a small team, too many sub disciplines involved for that. My favorite suggestion for starting is stuff like Arcanoid or Breakout to start with. Just get your feet wet, learn the tools and get a starting grasp on all the details involved.

Yea thanks both of you for the help I am aware that I am far from creating any type of mmorpg I was thinking of starting off with a text based one than move on to 2d maybe just to get some practice + experience. Hopefully in the long run I will soon be able to create a very basic MMORPG of course I will not be able to do it on my own...

I'd probably start on something simpler before taking on the mother-load with a MMORPG.

Start with a simple chat program.
Then, make the chat program accept more than one client.
Then, send the chat messages to who they belong to.
Then, handle out of order or non-arriving packets (if UDP) / handle split streams (if TCP)
Then, handle ungraceful disconnects.
Then, start to optimise packets for limited bandwidth.

That should keep you going for a couple of months. Then re-assess the whole MMORPG thing.