Одежда висит в шкафу - Yes, that's the way to go and шкафу is not in dative but in prepositional - висит (в чем? Где?) в шкафу.
Мой дядя интересуется астрологией - That's correct as well and that is not a specific construction, it's just that the verb интересоваться requires an object in instrumental.

Also: Этот сайт правильно? should be этот сайт правильный? Why? Because правильно is an adverb and you tried to modify the noun (сайт) with it, but for that you clearly need an adjective.

Я практикул с этим сайтом. Оно - практика для падежи - Практикул is not a word, also you refered to сайт with оно, but сайт is masculine so you should have refered to it with он.

Anyway, you managed to get your point across with your Russian sentences and that's what matters. So excellent job!