To write an amazing application, the most important thing to do first is to read the host’s and the position’s descriptions. Then you’ll know what kind of person they’re looking for and a little more about the activities you’ll be doing. This will also show the host that you’ve prepared well!
Talk about yourself: why you're traveling, why you chose that area and that specific host, where you've traveled already, etc. Basically, a brief summary about who you are and what you're looking for!
Then include a bit about your life experience: what you study or have studied, any kind of work experience (even if it’s not at a hostel/NGO/farm). Your life experiences will show the host several other things about you and how excited you are to take that next step.
Also, comment on the activities you will do: ask questions, let the host know that you are aware of everything and share how you can help him with his business.
But remember: there is a difference between asking some questions and making a questionnaire. Make sure that the information you need is not in the job description or even on the host’s website before you ask, okay? The best thing is to do a quick online search on the host before confirming anything. Good luck!