So, there's this tiny point of confusion some folks have, and it tripped me up at first too. The 'My Isekai Life' light novel series you're reading is actually the official English release title. The original Japanese title is 'Tensei Kizoku no Isekai Boukenroku ~Jichou wo Shiranai Kamigami no Shito~', which is a mouthful. The English publisher, Yen Press, just calls it 'My Isekai Life: I Gained a Second Character Class and Became the Strongest Sage in the World!' for the volumes.
If you're starting fresh, you begin with Volume 1, obviously. The narrative follows Yuji Sano, the salaryman turned monster-tamer sage, in a straightforward chronological order. The web novel, which came first, is a sprawling, rougher draft with many more arcs, but the light novel is the polished, official version you should follow. It hasn't caught up to the web novel's ending, so you're safe just going by volume number.
The tricky bit is side stories or extra chapters. Sometimes they're included in the volumes themselves, like interludes from the slimes' perspective or side events. There's no separate 'reading order' for those; they're placed where they belong in the volume. I'd advise against jumping to the web novel after the light novel unless you're comfortable with machine translation and a less structured narrative—it can get confusing with power scaling and forgotten plot threads. Stick to Volumes 1, 2, 3, and so on as they're released. I'm waiting for Volume 11 myself, the translation pace is decent but never fast enough.