The differences between various text types:
* Structure. An essay has an intro, a few paragraphs, and a conclusion. A feature article might have an anecdote, an intro, many paragraphs, and another anecdote to conclude. A letter has a date, an address (Dear..), a few questions, an explanation, a signoff. Make sure you write in the correct form.
* Audience. An essay may be addressed to a broad audience, or a specific subset of people (eg. Nuclear scientists). A diary is written to oneself or to an invisible 'other'. A speech is written to be read aloud to a group of people.
* Context. Essays are usually formal because they are in formal publications. Radio interviews are between two people but heard by many - so it has to be clear and concise (radio time is expensive!). A feature article is usually found in a magazine.
etc etc.
Keep these things in mind and you can tweak your essay to fit any of the given text types - you have to change your tone, your structure (long sentence vs short sentence), your word choice (scientific name or common name) and on the list goes.
hth
