I recorded my voice. And it's a quite long record for manually silencing out my breaths and lips clicking. I would like to make silence on areas where the sound is lower than some level. How can it be done?
Look for a 'noise gate'. I don't use Audacity myself and it looks as though my initial assumption that you'd have one built in is wrong: search for 'audacity noise gate' on google and you'll see results which suggest you need to get a separate plugin.
As Paul says, a Noise Gate is probably what you need in this situation.
What you are describing could also be done with an "expander" effect (so named because they expand/increase the signal-to-noise ratio, thereby allowing you to increase the volume of the track while the perception of noise is reduced). This is less useful if you don't need more Gain from the recording and you only want to reduce the more obvious noise.
I would not recommend manually editing out this noise in anything but very very short recordings. While that may be a good learning exercise to begin with, learning how to properly use a good tool will save you a lot of time in the long run.
Welcome to the world of dialogue editing. This is exactly the process that you will need to go through to get this edit right. Simply using an "expander" may get you part of the way, but will probably cause you more problems than it is worth - particularly if you are using audacity. The best way to deal with this is to persevere with a manual edit. You will need to check it anyway at the end. You will begin to learn to read the waveform. If you can use another software package, you will be better off. Audacity may be free, but it is very fiddly to use and will cause you more pain than it's worth.