Welcome! I recommend going for the cheapest/free mods to start with. Added tire pressure, weight reductions, dirty aero deletions are free ways to pick up a couple extra m'spg. Things like spark plugs and tires can make a small difference but aren't usually worth replacing for a more fuel efficient version unless you need to do it anyways. Make sure you read these 2 threads to get the most out of what you already have.
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
65+ Vehicle modifications for better fuel economy - EcoModder.com