Hi Connie, I’d be happy to help answer these questions.
1) The lack of volume is probably due to finger strength. Unfortunately, this is a technique that takes time to develop. Try to incorporate hammer-ons and pull-offs into your daily practice time.
2) The string should always remain straight, as in it should not be pulled up or down. It’s also important to make sure that you have proper pull-off technique (execution). This is discussed in detail starting at 3 minutes 16 seconds, in this video lesson: https://youtu.be/J6yCsCNRMW0?t=3m16s
3) Muting the other strings will help with this issue. If you look at the photo below, you can see that I am on a fingertip on string 2 (to the play that note). But my index finger is lightly touching the first string to mute it. This is an advanced technique, if you are a beginner player I wouldn’t worry too much about it just yet.
Please let me know if you have any questions.