Do you mean like a hardware board for educational purposes? I use the Sipeed Maixduino (or Maix Bit) for my classes. The problem with these boards is there is a lot of esoteric, non-documented items.

For a well documented hardware board, I would look at the Sifive Hifive1 (Rev B).

Since RISC-V is still relatively new, I'm sure more and more boards will start making it to market rather soon.

The RISC-V organization has a list of boards that they recognize here: https://riscv.org/exchange/

I think the easiest way to get started with RISC-V is to look at QEMU, which is an emulator. It can emulate the virtio bus, including graphics. I used this in my OSblog: http://osblog.stephenmarz.com which uses an emulated, 64-bit RISC-V CPU.

This book is a reasonable introduction to RISC-V:

http://riscvbook.com/

You mean what physical machine should you use to teach yourself on? I wouldn't bother trying to use any physical hardware - use a simple command-line simulator https://github.com/riscv/riscv-isa-sim.