There's a long delay when I plug my instrument into my PC. What can I do?

You probably forgot to install ASIO4All:

http://www.asio4all.com/

Once you've done that, to record with low latency:

1. Reaper > Preferences > Audio Device and select ASIO for Input Device and Output Device.

2. Track > Insert New Track.

3. Click the “AR” button to the left of the new track .

4. Click the Speaker icon to monitor yourself and select your input

5. Click the red circle button to record. You’re off!