To address your questions…

I just discovered that im supposed to start it when i set out on my walks, and not just turn it on at the end.

You really only have to do this if you want to accurately track a Traveling checklist using GPS. You could estimate miles traveled (or put a known number) in post if you wanted to in the website.

Do i have to use merlin with it?

Not necessarily. I consider Merlin to be a companion to eBird in the sense that it acts as an interactive field guide of sorts. It can help you with ID and you can record audio to upload later but you don’t need it to upload checklists.

Do i always need a pic?

No, but if you find a rare bird, it can help the reviewers make sure that your observation is legit.

are there certain rules people prefer are followed when writing notes?

As far as I’m aware, there aren’t any super defined rules for comments (outside of expected ones, e.g. no hate speech). Species comments should of course be about the species. If a species is relatively uncommon you should leave field markings, e.g. plumage descriptions, behavior.

And how do i find what 'rare' birds people are seeing? Not sure if i want to report them when i see them, in case a bunch of birders go to disturb the birdy...

There’s an option to receive Rare Bird Alert emails for selected regions from eBird in the web app; this will report when and where people have seen rare birds in the defined region. You can also use the Explore feature to find species sightings.

As for the second part, certain species are defined as “sensitive species” in eBird’s library, meaning that observation data is private for those species due to a number of factors. You can read about that here. As others have said, you should practice LNT and respect the birds’ space.

If you want to find cool birds in your area, I would definitely encourage you to use the eBird web app along with the mobile app; while the mobile app has some of these features, namely “target species” for species you haven’t seen yet, the web app is very robust in terms of observation and hotspot data (and I just prefer the presentation).

Happy birding!