The Raspberry Pi is truly an amazing little computer, and people have done some really good stuff with it. I have one, and it's great!
first, What os can blind people use with this computer?
The main operating system that powers the Raspberry Pi is Raspbian. You could get Orca running on it (don't know how well it works though, but I hear conflicting reports that speech crackles and stutters, but I could very well be wrong). In the end, it all depends on what you want to do with the Pi.
second, can i install os without help of sited people?
The Raspberry Pi's form of storage is a micro SD card, so you'd essentially flash a SD card image with any program you like (the most famous programs I know of for flashing SD cards are Win32DiskImager and Etcher on Windows). As for the initial setup, most of the time you won't get accessibility out of the box, so you will need sighted assistance.
and third, what speakers do i need for raspberry pi, eg with usb connection, 3.5mm jak, or raspbery pi have bluetooth and i can connect my bluetooth speaker?
To answer that question, if you're going to connect the Pi to an HDMI monitor you won't need speakers since the HDMI signal outputs both audio and video. However, since we're talking about external speakers, you can use the onboard 3.5mm audio jack for that, as I don't think USB audio will work, but I'd be surprised if it did. As for Bluetooth, if you are using a Raspberry Pi 3, it has onboard Wi-Fi and Bluetooth modules. You could connect your speakers, but since I don't use Bluetooth speakers with what I'm using my Pi for, I'm not sure if they will work.