We say "on a website". This is the preposition of place which generally goes with "website". So why did does it say 'at' in the second sentence? The answer is obvious if you think about it. 'At' is not a preposition of place in this sentence. The 'at' belongs to the fixed collocation "take a look at". You always TAKE A LOOK AT something.

A website is a situation where you can say either "on" or "at" I would say "on" is more common "on" = a bus, plane, train etc (transport) "on"= on a "hill, roof, mountain, tree, geological feature or a building. "on" = a table, chair,bed, refrigerator, a thing, "on"= the cat, a horse, the donkey, the elephant, animals. "on"= the river, the lake, the canal, other geological or man made features "on" the television, the movie, the internet, on the radar, man made inventions "at" at a location, can also be some of the above, the river, the lake, the beach, the mountain, sitting at the table, at the restaurant, but not on it, at the hospital but not on it, we tend to say "at" and not "on" when you do not usually expect to eat on the restaurant but in it, and go into the hospital but not on it to see the surgeon.

Thiago, thank you so much for your kind explanation! I was unable to understand through the example sentences on some language websites, but with your explanation, I get the idea now. For ex. '' At iTalki.com, you can find professional teachers for paid lessons as well as native speakers who give you great help to improve your language skills '' '' ON iTalki language website, you can practice your writing skills and meet many people to practice with '' Thank you!!

Want a piece of advice? When it comes to prepositions, often times more than one can be used. That's because there's a formal and an informal context in every language, and people usually end up using whatever sounds better versus the "correct" preposition. That said, ON a website is more common than AT a website. Also, I've noticed (I might be wrong) that ON is preferred when you use .COM .UK etc. Whereas AT refers (in most cases) to the name of the page alone. However, people would connect AT to the company's physical location as well. On Facebook.com At Facebook Many times they're pretty much interchangeable though.