Love this question, because it always seems too overwhelming for people initially. There's definitely some low-hanging fruit which I would focus on with something similar to the below order. Some great guides on here already. I consolidated a bunch into a SN directory the other day.
Separate passwords so never using same ones and have easy method to manage them - suggest Bitwarden but many solutions discussed here on SN.
Next, buy yourself a domain (see this SN post). And create a bunch of email aliases for each service you log-in to, so none share the same email. And neither the same password, because of #2.
Low-hanging fruit
Then, once have some momentum & motivation...