Fragrantica & Parfumo
Fragrantica is probably one of the most useful places to start. You can quickly check the notes, accords, release year, perfumer, community ratings, longevity, sillage, and user reviews. The reviews are especially useful because you can see how different people experience the same fragrance, but I would not treat every comment as a fact. Parfumo is another very useful source. In my opinion, it is especially good for checking performance ratings, fragrance classifications, and more structured community feedback. Comparing Fragrantica and Parfumo together can already give you a pretty solid first impression.
Communities & Personal Opinions
YouTube, Reddit, and fragrance communities are also useful when you want more personal opinions. These are better for questions like “How does this actually smell?”, “Is it worth the price?”, or “How does the newer batch compare to the older one?” The downside is that personal opinions can be very different, so it is better to look for patterns instead of trusting one person completely.
The Brand's Own Website
For official information, the brand's own website should always be checked as well. This is usually the best place for the official note list, concentration, perfumer information, bottle sizes, and how the brand itself describes the fragrance.