It depends. If you're reading manga on legitimate websites that have the rights to distribute it, it's not illegal. However, if you're getting it from illegal torrent sites or unlicensed platforms, then yes, it's against the law.
The legality of reading manga online depends on the source. Legitimate platforms with proper authorization make it legal. But most random sites offering free manga are likely illegal and can get you in trouble.
You can start by looking for websites that offer Japanese manga with translation options. Some popular ones are MangaReader or KissManga. Just make sure they're legal and safe to use.