The six basic tools for manipulating sound are found on the left hand side of the tool bar. They are the select tool for selecting sections of sound, the zoom tool for zooming in on an area of audio, the envelope tool for manipulating the volume of a sound sample. The draw tool for manipulating its amplitude, the time shift tool for moving samples relative to each other and the multi tool which does all of the above depending on the key modifier you press.
Deleting part of the audio
Step 1: Select an area of sound that you want to delete by left clicking where you want the sound to start, holding the left mouse button down and dragging the cursor across. You can change the boundaries of an already selected area by moving the cursor over the edge of the sample.
Step 2: After Selecting the audio to delete, click on play (while the selected is highlighted), this will play the part of the audio that you want to cut. After you confirm that, click on Edit > Cut to delete that audio section.
Step 3: Go to File > Save Project to save your project.
Combining multiple Audacity projects together
You could combine two of your Audacity projects or mp3 files together to export/save them as one file.
Step 1: Open Audacity. Click on File > Open and then select the first project/mp3 to open.
Step 2: Go to File > Open again and select the second project/mp3 to open. You will notice that this file will open in a new Audacity window.
Step 3: Click on the file window that you would like to copy. Select the entire audio and go to Edit > Cut.
Step 4: Go to the file window that you would like to append the audio to. Put the mouse cursor towards the end of the audio and then go to Edit > Paste.
(If you want to have some silence noise in between the two audio files, before pasting click on the end of the audio and go to Generate > Silence. Enter the length of Silence in Seconds and then click Generate Silence).
Step 5: Click on File > Save As to save your new project.
Audacity also comes with a number of built in effects. These allow you to change the volume of a track, vary its pitch and tempo independently of each other, add echo, reverse a sample etc. You will need to have some audio selected for them to work.
Step 1: Select the audio that you want to apply the effect to.
Step 2: Click on Effect and apply the effect you would like to use.