The topic on how to build a questionnaire is not trivial and it is teached in psychology courses. It is hard to summarize.
Golden rule: have clear in mind that you have your own personal belifs about any topic (for example why you, or the users, want to download an app). You should be aware of what is your opinion about this topic and try to considerate any other possibility.
I would start with open interviews. The aim of these interviews should be to let users tell what is their reason why they downloaded. Be careful not to look for a confirmation of your opinions, not lead the conversation in the direction you suppose it should go. Just ask direct and simply questions, be open to any kind of reason and basically be curious about people's motivation (you could be surprised!). Do not comment the answers, create a relaxing setting and be supportive and sincerely curious. Be careful about what you say (the lesser the better), use non verbal language to be supportive and welcoming.
Once you collected many opinions and ideas, you should summarize in concepts (for example: money saving, health, fun, "just for try", etc.) and then build a questionnaire using a Liker scale on these concepts. The questionnaire would add quantitative validity to your qualitative data collected with interviews.
Hope this helps!