If the site is yours, Google Analytics s your answer. As a certified GA user I can tell that the tool can be cumbersome for a beginner, but there are lots of great tutorials, but the only source you'll need is Google Analytics Academy.
If you're conducting a research of a niche and analyzing competitors, then you'll bound to use 3rd party tools. If it's just traffic figures that you want, just use SimilarWeb - it does a fair job "estimating" the received traffic, but the real figures won't be seen unless you hack competior's site. If you need a more detailed analysis you can use tools like Serpstat, Ahrefs and Semrush. These can estimate the organic traffic received by each page.