Your question is getting downvoted because you're asking an extremely large scale question that comes across as "How do I build my app". There are millions of ways to answer this question, but the decisions need to be made by you. This app CAN be made by a beginner, if you're patient, persistent, and resourceful. Break your larger problems into smaller segments and learn those one by one. Something like this would be a way to break down your problem.
Step 1: Google "How to find user's location in iOS"
Step 2: Google "Gas station api" this was literally the first result:
http://www.mygasfeed.com/keys/api
Step 3: Google "How to make an HTTP request in iOS"
Step 4: Google "How to use JSON in iOS"
Step 5: Put them all together.