How can a 14 year old earn roughly 500 dollars within a few months if the money would go towards a homemade air balloon that I don't want to share?

How can a 14 year old earn roughly 500 dollars within a few months if the money would go towards a homemade air balloon that I don't want to share?
Depending upon what is meant by not wanting to share the balloon,
one way might be to work out the total visible area of the finished balloon and divide this into oblong or square panels. When you know how many panels you have available, contact local firms offering to feature their business on an advertising panel which will provide a unique highly visible and mobile platform to promote their services. An editorial feature in the local press would provide a free way to attract advertisers and getting exposure on the internet would attract a great deal of interest adding value to your offer. Price the panels at a rate that will provide the money you need to raise in total with a margin left over for profit and there you go. Along the way, you have also become an entrepreneur!

You still own the balloon so don't need to share it!