Mastadon is to Twitter as Lemmy is to Reddit. So you don’t really follow topics, but you can follow people if you want (I never have) and you can join communities on the topics you’re interested in. Some of the things you’ve mentioned, like Linux, are very popular/prolific here.
My advice is to set your browsing by All (and whichever of the other one makes sense to you; I usually do New, but sometimes one of the Top ones), then when you see something posted in a community that you’re interested in, instead of clicking on the post, click on the community name. In the sidebar, you can subscribe to the community. After you’ve been doing that a while, if you want you can change your browsing to Subscribed.
There’s also a Lemmy Explorer, and you can set it to Communities at the top and find the ones that are the most active or are on the subjects you’re looking for.
Having your posts seen is mostly a matter of posting to active communities with engaging topics.