If you are not given a detailed assignment, use your understanding of your writing situation to choose your genre.
Spend time thinking about your purpose and role, and then ask yourself which genres might best help you accomplish your goals.
Reflect on the knowledge, interests, purposes, and backgrounds of your readers, and then ask what this might tell you about appropriate genres. Try to understand what your readers are likely to expect from you and what they might find familiar and easy to read. A well-written scientific report, for example, might be less effective than an informative essay or article simply because your readers might be unfamiliar with the scientific-report genre.
Reflect on the context in which your document will be read. If you know that your readers have easy access to computers, tablets, or smartphones, you’ll have more options than if you know that they can read documents only in print form.