You should look at some of the topics under the 'industry discussion' section, e.g. producing some samples. Dig in and you may find some useful info. As your question is somewhat general, I will try to give you some basic input.
First thing first (and extremely important) - do your market research. There are multitudes of beverages available on the market, so you need to introduce something that is new and refreshing ('no pun intended') that will captivate your target market. Don't waste your time with 'me too' beverages, as the big 'evil' beverage corporations are already busy spending (or wasting) money in that aspect.
After (if you're still keen) - work with a lab to develop your formulation and produce samples ($$). This is necessary to approach a co-packer (unless you want to produce your beverage out of your kitchen) and helpful to contact distributors/retailers.
I wouldn't suggest contacting existing companies with just ideas, at least not until you have a finished product, formulation in hand, package and all.