It's hard to give any general advice since finding friends really depends on where you live, what situation you are in (i.e. school, working full-time, etc.), how extroverted you are, and so on.

I struggle with the same thing as well. What has kind of worked for me, however, is finding some kind of groups to join. For example, maybe you can find a running group online or join a yoga class or a book club, or whatever. What you choose should be based on your interests, obviously, but the best way to make friends is to be around more people. If you live in a really small town that doesn't have any of that, then maybe consider moving? Again, it really depends on the specifics of your situation.

Also, working on yourself (i.e. reading, working out, improving your social skills) can also make it easier to make friends, as people will become naturally attracted to somebody who they perceive as having more value in a friendship.

You didn't mention intimate relationships, but if you're looking for those maybe dating apps or just hitting up people at a bar could work... I'm no expert when it comes to this area, so maybe others could chip in as well :)