how do I modulate different volumes in a string..?

I know very little about sound and about Audacity. My problem is I have six different MP3 files I have to string together - and they must all end up with much the same volume.

At the moment they vary quite a lot. How can I reduce the noisier ones to the same optimum level but also make them all sound natural. I know this is probably an incredibly basic problem for most of you but as I said, I’m a complete novice…

Anybody help…?