Translation:I want something to drink.
I'm a little iffy with translating every such to "want". The Japanese are ever so polite even in general everyday conversation, and here we come with what can easily come off as a demand instead of a polite request. Why can't this be "would like" instead, especially with desu also tacked on at the end?
飲み物 nomimono - (noun) drink/beverage
飲みます nomimasu - (verb) to drink
It's most literally saying "I want a drink (beverage)", which to many English speakers sounds like the person wants an alcoholic beverage, so rather than the "something" being implied, it's there to make the translation clearer I think.
I want to drink would be 飲みたいです (nomitai desu).
物（もの） just means "(physical) thing". のむ means "to drink", while のみもの means "a drink".
The technical term for turning a verb into a noun is "nominalize", and the words used to nominalize are "nominalizers". In Japanese, the usual nominalizers are こと "(non-physical) thing" and の "one". もの is only used in very specific cases, like this one. I don't think it's considered a nominalizer; のみもの is just like a compound word in English, and you should treat it as such.