Due to the decentralised nature of Mastodon, finding people to follow may be more accessible on large Mastodon servers, but smaller ones may require some looking around outside your server. In particular, if you are joining a new or smaller server, that will be the case more often than not.
If you have just installed a brand new Mastodon server yourself, I recommend reading the first steps after installing a Mastodon server guide I have created.
If you already have a Mastodon account on another server, you can move the account by following the move account instructions on Mastodon documentation, or you can export your list of follows and import it on your new account Account->Import and Export.
Follow people from your local/federated timeline
The first place to look is on your own server. Following the administrator of your server would be a good idea as they may announce relevant content there. Plus, checking the local timeline for other members on the same server may also be a good idea. The federated timeline will include remote users that federate with the server you are on, and you can find some people to follow there.
Check other users follow lists
If you know someone on Mastodon, a good idea is to look at the following/followers lists of that person. For example, you can look at Masto.host Mastodon account following or followers. Then, when you find someone you like, you can follow them and then look at that user following/followers.
View other servers /public and /explore pages
You can also go to another Mastodon server public page (example: https://fosstodon.org/public) or the explore page (example: https://mastodon.online/explore) and find more people that way. You can find a list of Mastodon instances and other servers with ActivityPub compatible software on instances.social.
Look for users by category on Trunk or the Fedi.directory
Another option is to go to Trunk. Trunk is a directory of people on the Fediverse, where users can submit their profiles on topics they post about.
Have a look at the categories that might interest you and look at the profiles listed to decide who you would like to follow.
If you are just getting started and without much content on your profile, it’s probably not the best time to submit your profile on Trunk, as the Trunk admins will have little information to know if they should approve your submission.
There is also Fedi.directory where you can find a curated list of follow recommendations per category.
Batch follow users from categories on Trunk
If you don’t want to manually go through Trunk categories, you can go to https://hugo.gameiro.pt/trunk-to-csv and pick the categories from which you would like to follow everyone.
That will generate a CSV file you can import to Mastodon to automatically follow all the users from the categories you choose. On Mastodon you can upload the generated CSV on Account->Import and Export->Import.
The FediFollows Mastodon Account
FediFollows is a Mastodon account that regularly shares recommendations for people to follow. You can find it at https://mastodon.online/@FediFollows.