There are of course some restrictions to this Government scheme. You will have to be at least 16, a UK resident and not own any property anywhere in the world to open an account. Then to get the Government bonus, the home you're buying needs to be in the UK, cost no more than £250,000 (£450,000 in London), be purchased with a mortgage and not be rented out (though you may be able to change your property to a buy-to-let investment later down the line).
If you've already opened a cash ISA in the current tax year (which runs from April to April), you'll only be able to transfer those funds, and even then only up to £1,200, into a Help to Buy ISA; you won't be able to open an additional active account. This is because you're only allowed to open one of each type of ISA per year, and Help to Buy ISAs are considered the same as cash ISAs (and largely follow the same rules).
If you've put more than £1,200 into the new cash ISA, the extra funds would need to be put into a stocks & shares ISA (as you are allowed to get both a Help to Buy and stocks & shares ISA in the same tax year), an innovative finance ISA, a stocks & shares lifetime ISA or a non-ISA account. ISAs you've opened in previous years do not count, though you won't be able to save more than your annual ISA allowance across all your ISAs. For the 2018/2019 and 2019/20 tax year, this allowance is £20,000.