To test a scene you've created in Unity in VRChat, you need to first log in with your VRChat account by going to the VRChat Menu and then selecting
VRChat SDK > Settings. Once you're logged in, make sure your Installed Client Path is correct and pointing to VRChat.exe.
Now go to
VRChat SDK > Show Build Control Panel. If it asks you to add a scene descriptor go to
Assets > VRCSDK > Prefabs > World, drag VRCWorld into your scene, and place in the location where you want the player to spawn.
In the Build Control Panel there are 4 options. To test your world in VRChat locally without uploading what you want to select is
New Build under the Test section. Once you click this button Unity will build a VRChat version of your scene and then launch VRChat and load your world.
There is currently no way to test an Avatar locally. To test an Avatar, upload it to your account and select it as avatar in-app.