Anyone over the age of 16 who is resident in the UK can open a cash ISA. However, you will only be able to open one cash ISA per tax year, and this includes Help to Buy ISAs. So, if you're just starting out, you'll have to decide whether you want to get a fixed rate cash ISA first, or an easy access version. Some providers allow you to invest in both fixed and variable rates within the same ISA account which can give extra flexibility; check the product details closely.
Any cash ISAs opened in previous tax years are irrelevant, as you can have as many ISAs as you want – provided you adhere to the general ISA restrictions. Note that you can open a cash, stocks & shares, innovative finance and lifetime ISA in the same tax year, as long as you don't exceed the overall ISA allowance, as these are considered separate ISA types. A Help to Buy ISA, however, is considered a cash ISA.
Once you've decided on the best fixed rate cash ISA for you, simply contact the provider to find out what you need to do. The providers' permitted account opening methods are clearly displayed on our results section, and some accounts may even have a orange button that makes it easy for you to click through and apply.