http://strategywiki.org/wiki/Kid_Icarus
is a very detailed guide. You might want to skip the walkthrough, unless you don't mind, but its details on what items are and what they do and how to get them are invaluable.
One thing that I didn't find in the guide that helped me a ton though:
Reapers are not dangerous enemies. Reapettes are not dangerous enemies. They are very useful. In the first two worlds, all reapers are on platforms so they won't run up and attack you. Also, Reapettes will follow the exact same path every single time that they enter the screen. You can use this to your advantage by positioning Pit in such a way that every time the Reaper calls the Reapettes and they fly down into the screen, you just have to hold the B button for a moment to fire off 4 shots and kill all of them. Then, the Reaper calls out for them again, and you just keep repeating this process until you have the 20k, 50k, 100k, 200k points you need to get the Life Bar upgrade at the end of your level, AND enough hearts to buy whatever you need, AND enough kills to get the Arrow of Strength upgrade for whatever level you're on.
Because the Reapettes are 100% predictable, its incredibly easy to take 30 minutes to kill 300 or so and rack your score up to whatever you need it to be in the first 3 levels. By the time I got to the boss in 1-4 I had 2 extra Arrows of Strength, 3 extra life bars, a barrel with 2 potions in it, 2 feathers, and the Fairy Orbitars. I tried to get enough hearts and points by just shooting every single enemy, and beyond getting hit a ton of times I often would come up short even if I killed every single enemy in the level. This way, I just dodged most enemies I came across, unless I couldn't, and just got the points and hearts and kills I needed for all the fancy upgrades at the reapers. Since I took the time to grind so hard in the first world, all the levels since have been pretty easy. Once you take the ridiculous difficulty out of this game, its a pretty enjoyable romp kind of like MariomeetsZeldameetsMetroid, shooting enemies, jumping across platforms, dodging attacks, choosing the best item to buy at a shop.