Declining loaned words sometimes can present a problem even for a native speaker. But Russian grammar hasn't changed cosiderably in the last ten years, so don't worry.

First, table of cases:
Nom. - джинсы
Gen. - джинсов (some people can use an incorrect form "джинс")
Dat. - джинсам
Acc. - джинсы
Instr. - джинсами
Prep. - джинсах

And now some rules:
1.
The word "джинсы" is always in plural, that means that in Nominative case it should be used with the collective numerals ("собирательные числительные"), i.e. двое, трое, четверо, etc., and in oblique cases "ordinary" numerals ("количественные числительные") are used: "о трех джинсах" (not "о троих джинсах").

In real life many people break this rule, because as I said loaned words are sometimes tricky and in combination with Russian grammar they can cause a confusion. So people go for the form, they consider to be "more natural", disregarding the rule.

2.
Also "джинсы", along with some other garments (trousers, socks, etc.) can be counted in pairs, i.e. "одна пара джинсов, две пары джинсов, ... пять пар джинсов", etc. (the same as in English, I think).
And imho "у меня пять пар джинсов" sounds better than "у меня пятеро джинсов".