My mind boggles as to why someone would downvote this question. Why not just help, ffs?
I run a self-hosted instance, so I have some experience with what you're asking. You do not need to sign into every instance to post or comment. Basically, you need to first find the desired community using the search function in your own instance. Once you've found it, you can view, post, or comment through your own instance. This is what Federation enables.
For example, let's say I want to view posts or comment in a community over at lemmy.ml using my own instance. Let's say the community is called c/asklemmy. As long as federation is enabled on both instances (which is the responsibility of the instance administrators), you can use the search function, within your own instance, to find the community and interact with it at your leisure.
When searching for a community on another instance, remember to specify that the community is on another instance. So, for asklemmy over at lemmy.ml, you would search for [email protected]. The format is !(community name)@(instance domain). You can even copy and paste the URL of the community's page into the search bar, and it will work just fine.
There's documentation available at https://join-lemmy.org/docs/administration/federation_getting_started.html that explains all of this in more detail. Anyway, I hope this has been helpful, and welcome to Lemmy! (It's so much better than Reddit).
P.S. If you’re looking for an iOS client for Lemmy, Memmy is really good.