How i add a background noise in one spot of a song already playing song/vocals?
Load the song in Audacity and load the sound. The sound will appear below the song and they will both play at once. Use the Time Shift Tools (sideways black arrows) to shift the sound rightward until it appears in the right place with respect to the song. Then use the volume slider on the left of the sound and the fade in and out tools, etc to make the sound match.
When you export, the sound will be married into the song.
Koz
Please note that Audacity doesn’t edit MP3. If you try to do production in MP3, the honky compression distortion will go up when you try to make the new song. That and you can’t make an MP3 at all without installing the optional lame software.
Koz
Ok thanks about the sliding part. 
Ok, I cannot get it to work. I do have the newest version also. Please help!
Why not? What does not work? 
It just stays where it is. I guess I am not setting it correctly in the spot I need it.
Use the Time Shift tool to drag it to where you want it. Playback must be “Stopped” (not “Paused”) to use the Time Shift tool.
Then do I save as ?
To create an ordinary audio file you need to “Export”.
You will probably find it useful to have a look through the topics listed in the manual here: http://manual.audacityteam.org/o/quick_help.html