I have been in research most of my life. Depending on your field, you should look up for a thesis supervisor that falls within your research interest.

I'll give you an example. I was a microbiology graduate. After which, I embarked on my research dealing with antimicrobial, which mainly focused on searching for new natural antimicrobial compounds. I have pretty much learned everything on bacteria, which later led me to pursue my PhD on studying virus.

Basically I didn't deviate much from my undergrad. Just that I moved deeper into two main microorganisms, which are more related to human health. Of course there are others too but I decided to pick up these two as my core.

So it's basically up to you to decide what you want to do.