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.

There's other blind community answers you can find over on [Raspberry VI], in general Raspbian Lite is used which doesn't have a visual interface, as its largely considered redundant and isn't accessible out of the box anyway. Typically users SSH in to the raspberry pi either through Wifi or an ethernet cable, when flashing your SD card you can setup both SSH and the Wifi on the card before plugging it into the raspberry pi. You also won't need any sighted assistance for this.

As for audio, as FamilyMario mentioned the HDMI covers audio, although you'll also need an HDMI compatible monitor to do that. The 3.5mm audio jack is a reliable source, and as for Bluetooth... The Raspberry Pi does have Bluetooth and Wifi support, but there have been issues related to Bluetooth audio in the not too distant past. I think they found a work around, but its may not work out of the box.