How to make one file of mp3 with several ?

I’m a new user of audacity and know nothing about it. I have on my computer about 4 hours of songs cut in about 40 mp3 files, and would like to make of this just one file lasting the same time, keeping the sound as it is, and save this one file of about 4 hours on my computer. Can someone explain me step by step ? The following is the same question in French.
je suis un nouvel utilisateur d’Audacity et n’y connais rien. J’ai sur mon PC environ 4 heures de chansons découpées en 40 fichiers mp3, et voudrais n’en faire qu’un seul de la même durée avec le même son, en sauvegardant le fichier final sur mon PC. Quelqu’un peut-il m’expliquer pas à pas comment m’y prendre ?
Thanks to the one who can help me !
Merci à qui saura m’aider !

It’s better to forget about Audacity if you want to make an MP3 out of the shorter MP3’s, because Audacity will have to re-encode the final MP3, which will make it worse quality than the original MP3’s.

Instead, use one of the tools listed here that can join MP3 files without re-encoding: . They are not our tools, so ask the makers of those tools for help if you need it.

Those tools assume that the MP3’s have the same sample rate, number of channels and stereo or mono encoding, which is probably the case if some tool split them up in the first place.