I'll try to answer to the best of my knowledge, though I'm sure other crafters will give you better advice.

1. If you're still levelling, any cheap food will do. I'd go for stone soup from the gold saucer for the bonus to CP.

2. Once you unlock collectibles there are better engineering manuals to buy from scrips, but before that I don't think so. There's also one type you can get from squadrons once you unlock them for a flat 20% bonus over 2 hours. Other than those, yoi can try asking your FC leadership for the company action that gives bonus crafting xp.

3. Try looking for Camie's guides. They may be a bit outdated but otherwise they're great. For general levelling advice, do GC turn-ins, leves, either repeating or travelling ones, and once unlocked beast tribes and custom deliveries.

4. If you're keeping you gear updated, go for the most CP you can get, it will make the biggest impact.

5. At low level observe can be useful to skip a poor quality step after an excellent, but it's otherwise not really worth it. Later you'll learn skills that combo off of it and it will get much more interesting.

6. BSM is useful since it makes the tools for most professions, and the rest are crafted by professions needing BSM tools. Most of the gear will come from WVR. And in general it's a good idea to level everything at roughly the same rate to get cross-class skills, though that's less important these days. Until you unlock better skills, Hasty touch is your unreliable best friend, if combined with Steady Hand (II).

Hope this helps!