You can say either 'She has long, dark hair' or 'She's got long, dark hair'. There is no difference in meaning between the two. The first is a little more formal, and more typical of written language. The second is actually easier for us to say in a rapid natural way, so it's typical of the spoken language. Note that we don't usually say 'She has got..' : we nearly always make the contraction 'She's got..' 'Jeans' (like trousers, pants and so on) is a plural noun, so you can't say 'a jeans'. You have to say just 'jeans' or 'a pair of jeans.' If you say 'He has jeans', we'd understand this to mean that he owns a pair of jeans, or that he has jeans in his suitcase, or in his wardrobe at home. If you are describing someone's clothes and appearance at the moment, you'd say 'He's wearing jeans.' I hope that helps.

She has long, dark hair . She has got long, dark hair. Both of these are fine. Jeans are uncountable, the same as trousers. To make it countable you need to say 'a pair of jeans' or 'two (or more) pairs of jeans.' 'He has jeans' means he is in possession of some jeans, not that he is wearing them. 'He's wearing jeans' means he has them on his body. 'He has jeans on' means the same as 'he's wearing jeans.'

I think you should say "He has a pair of jeans"