Should kill most of the worlds population leaving a few people to restart life.
That's easy enough. As the author, you can simply declare this to be so.
Must have a herbal cure/vaccine so that the protagonist can meet someone.
A "herbal vaccine" isn't a thing; vaccines are made from weakened, inactivated or dismantled infection particles. They are also generally the end-product of a fairly sophisticated research and industrial process, so if one did exist for your virus it would have to have been made before the outbreak. There's a conspiracy hook for you.
A naturally occuring herbal cure also sounds slightly dubious, because the timescales involved make it largely impractical to discover in time to help anyone. If it had been discovered before, you've got another nice conspiracy hook. It might also facilitate the presence of groups of survivors who were already familiar with the herb and had been cultivating it for other uses (medicinal, recreational, agricultural, whatever).
Must spread all over the world at a speed that the news and governments don't have enough time to fully report the problem.
You'll have a real problem with this. You can get realtime news updates from anywhere in the world if you have an internet connection these days. You might perhaps be able to handwave something that spreads silently and effectively with a long incubation period, then suddenly activates and kills everyone in short order.
In Oryx and Crake
a bioweapon is hidden inside a wildly popular pharmaceutical product, allowing its dissemination across most of the planet before it was activated and spread "naturally".
In The Stand,
once the superflu (also a deliberately engineered bioweapon) outbreak becomes apparent it is deliberately spread by military agents already sited in foreign countries to ensure that everyone suffers from its creator's carelessness.
There are many other fictional examples you could take; this is a very, very common trope. In The Day of the Triffids, for example, the titular hazards (which may or may not be bioweapons) spread far and wide long before a separate catastrophe prevents humankind's ability to keep society together, at which point the triffids become a massive and dangerous threat overnight. And so on.
It could be waterborne but then I wouldn't know what to do with the need to drink liquid.
Modern water treatment is extremely effective at killing and filtering things down to the size of viruses, so this mechanism of spread seems likely to be a poor choice of infecting a modern society in a reasonably developed country.
Stick with the traditional means of spread, they work well enough. Measles is perhaps a good model, as it is extremely contagious and is spread in droplets that can survive on exposed surfaces outside of a host for several hours. The Stand uses a souped-up version of influenza, another favourite that can spread quickly and has been known to cause deadly epidemics.