First off, head on over to Generate Randomize Game and provide your ROM.
If you'd like more detail on what the flags do, check out the Flag Explanations page.
If you haven't played Super Mario RPG in a while, you should check out the Player's Guide page to jog your memory!
Once you've got a randomized ROM, you need a way to play. There's a couple options for that...
An emulator is a program that closely replicates SNES hardware, allowing you to run SNES games on your computer.
While you can play using only your keyboard, a controller makes for a better experience. While most USB controllers will work, the most recommended ones include the iBuffalo Classic USB Gamepad or the 8Bitdo SFC30 Wireless Bluetooth Controller.
Some emulators available:
Yes, you read that right! You can actually play this on your original SNES console now. Playing a randomizer on original hardware requires a "flash cart", which is a special cartridge designed for that hardware that allows you to put the randomized ROM on some kind of portable flash memory (usually an SD or MicroSD card) which is inserted into the flash cart, which then goes in the original console.
The best SNES flash cart on the market right now (and the only one that currently supports Super Mario RPG) is the SD2SNES. You can learn more about it, download the firmware, and find a shop to buy one on their website here.
Until very recently, no SNES flash cart on the market supported the SA-1 enhancement chip that SMRPG uses. However thanks to the insane work of RedGuy, SA-1 support has been added to the latest SD2SNES firmware!
Make sure you're on the latest official firmware!
If you don't have an SD2SNES and don't want to play on emulator, it is possible to play this on the Wii Virtual Console. Prior to RedGuy's recent work, this was the preferred way to play as close to original hardware as possible. This requires locating a WAD file of the North American release of SMRPG on the Wii VC. The randomizer can pack the generated ROM into the WAD file for you automatically.
Once you have the packed WAD file, you can install it on your Wii using the Homebrew Channel. Setting this up is beyong the scope of this site, but we do have some instructions on the community wiki that can be found here.