1. A free website
Search for "text to MP3" and you'll find plenty. You paste your text, pick a voice and download an MP3. Quick, and nothing to install.
The catches: most only take a few thousand characters at a time, want an account for more, and send your text to their servers, so don't paste anything private. Free plans often don't let you use the audio in videos you publish.
2. On a Mac, free and built in
Open the Terminal app and type say -o speech.aiff "Your text here". That creates an audio file called speech.aiff. For a whole text file, use say -f mytext.txt -o speech.aiff. It uses Apple's voices; better ones can be downloaded in System Settings → Accessibility → Spoken Content.
3. On Windows: Balabolka, free
Install Balabolka, paste or open your text, and use the File menu to save it as an audio file, MP3 included. It uses the voices installed in Windows, which sound fairly robotic.
4. VoicedByMe, natural voices with no limits
Paste or type your text, or open a document, pick a voice, and click Save audio. Then download it as an M4A or WAV file, or copy it straight into a chat. M4A plays almost everywhere MP3 does: phones, computers and messaging apps. A whole book becomes an audiobook with chapters.