First, some vocab.
- Dashboard = feed
- Staff = admins
- Blog theme = custom look of a blog
The default "For You" dashboard is kinda trash. It's one of tumblr's attempts at an algorithmic feed. It mixes stuff from your following with stuff from tags you've liked, but doesn't do a great job at either. If you want a pure reverse chronological feed with no algorithm, I suggest you use the "Following" tab. If you want to actually see new stuff from your interests, then I suggest you go to the Explore page and then use the "For You" tab that is there.
Reblogging works just like a re-tweet, except that if OP deletes the original post, all reblogs stay up. Think of reblogs as copies, rather than direct sharing. That also means that very old posts are still circulating around today. Which is cool.
There are bot waves on there. It's not as bad as it sounds though, they're mostly mildly annoying by giving you useless notifications. You might not see them for a while, since they tend to target more established accounts, but if you see one, the rule is to report them as "spam and/or bot". The more we report, the easier it is for Staff to figure out the current pattern. Currently some bots are countering this by having no bio and no profile picture, so if you want to avoid being mistaken for a bot, please set one up.
Your likes are public by default and will be shown to your followers right on their dashboards. If that's OK with you, carry on, otherwise you can make them private in your account settings.
Speaking of settings, there's actually a lot that you can customize in there. Don't hesitate to spend some time going through all the options!
And speaking of customization, if you're on desktop, the most popular add-on for tumblr is XKit Rewritten". I can't recommend it enough. It's like the YCC of YouTube or the RES of reddit. But on steroids.
And, while we're on the customization topic, you can actually fully customize your blog. I'm talking like, full HTML and CSS and Javascript customization. So go hogwild if you want! However please note: blogs can have 2 different views. The default one is basically a minimalist view that only has the posts. You can find that one at www.tumblr.com/name. If you want to actually see a blog's custom theme (if it has one), you need to go to name.tumblr.com.
When you create a post, the tags you put will make it show up in tag searches. If you put tags that not relevant to the post (like on Instagram where people put all the trends in their tags just to be seen), that is actually against tumblr's ToS and you can be reported for spam. The only exception is for nonsensical tags, or "Commentary Tags" which brings me to the next point
Tags on tumblr are a new form of communication (not a joke, there's been research on this lol). You will often see people put whole sentences in there. That's fine. These are essentially comments from the poster (or reblogger), and are seen as the acceptable form of commentary when there is nothing to add to the post. Embrace it, it's very fun, especially if you're neurodivergent. I could elaborate but I won't.

These are all the big tips I can think of right now. I probably missed a lot. If you have any question on specific parts of tumblr I would love to answer those.

Finally as for recommendations, well, I don't know your tastes, but if you're into cool girls, furries, tech, and cool things in general, then I suppose you would enjoy what I post here.