Due to the decentralised nature of Mastodon, finding people to follow may be easier on large Mastodon servers but on smaller ones may require some looking around outside your server. In particular if you are joining a new server or smaller server that will be the case most often than not.
If you have just installed a brand new Mastodon server yourself then I would 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. Probably 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 have a 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 explore page (example: https://mastodon.online/explore) and find more people that way. You can find a list of Mastodon servers and other servers with compatible software on instances.social.
Look for users by category on Trunk
Another option is to go to Trunk. Trunk is a directory of people on the Fediverse, where users can submit their profile on topics that 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.
Batch follow users from categories on Trunk
If you don’t want to manually go through Trunk categories you can go to hugo.gameiro.pt/trunk-to-csv and pick the categories from which you would like to follow everyone.
That will generate a csv file that 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 of people to follow. You can find it at https://mastodon.online/@FediFollows and they also have a thread with a curated list of follow recommendations per category at https://mastodon.online/@FediFollows/107547349637668904.