many devices (neighbors and such) have connected to my wifi in the past, now it feels like im not the only one using it, any tips?

most routers have MAC Address filtering. whilst in some cases it might be possible to get around that in most cases it will lockout any unwelcome guests. it works by having a table. this contains a list of all the devices you want to ALLOW to use the wifi. you then have a SWITCH which is the ENABLE the filter option in the routers menu. once switched on, only those devices in the list can connect. there are other minor details which should be used too. the option to FIX that devices IP. so that usually appears when you ADD a device to the list and it would say something like Always Use this IP for This device which you should tick. so connect a pc via a lan cable to the router and LOGIN. the user/password should be printed on the label. find the Connected Stations List(the devices currently connected) this will list each devices MAC Address and is the way you check WHO is connected at any moment and helps if you need to write down the MAC for a device before adding it to the filter table. routers vary in how the stations list can be used to add devices to the filter table and may or may not have a button to do that there helping you to not need to write down the MAC address. below is one example of a netgear mac filter table and how to add a device thats already connected. you can operate the table in a reverse way and list devices you want to prohibit, but its not as good a defence as you would need to keep adding more and more. any way the link below should give the idea

tighten down security; change the password to a much more complex one; implement MAC address validation

Your router has two passwords, one for the Admin to access the router's settings and another for accessing Wifi. Disconnect the router from the internet then change the Admin's password first then change the Wifi password. Never ever share those passwords to anyone. The only way to break those is to reset your router by pressing the tiny button at the back so secure your router at all cost.

Change your password or setup Mac filtering

You can start by changing the password and making sure you don't share it. You should also be able to connect to the router and see what devices are connected to it and boot those you don't recognize.