How can I baked GI with good quality, less noise and good shadows

All the objects that don't move and that you want to be included in the bake need to be marked static.

You need at least 1 directional light set to mixed or baked, and casts shadows. This light should not change during the game or be animated at all. If you haven't baked yet, you won't see a change when you change the light mode.

In the lighting window, Set your ambient light color and intensity or use a skybox to determine the way ambient light (light that comes from everywhere at once, or non-directional) is going to effect the scene.

For quick bakes, resolution sizes like 2 or 4 texels should be fine The final bake should be more. The default is 40 texels I believe.

Ambient occlusion darkens corners where mesh comes together to give more depth. Final gather takes a long time but produces better quality. Max distance is how far away from the corners the darkening starts. Higher Ray counts increase precision. If you don't like the baked AO, you can also use Screen-Space AO on the camera as an image effect.

In my experience, combined meshes just don't play nice with baked shadow maps- somebody else will have to advise you on that.

When you bake, you will notice that your non-static meshes no longer cast shadows on static meshes. This is a problem to be fixed in a later release.

To get around this, you can use a blob shadow projector to make quick shadows for your non-static meshes. OR, a more computationally expensive solution is to set your baked light a little dim, and then use a second realtime light that casts from the same angle to get your non-static shadows back.

Pre-computed lighting will give that realtime light the ability to bounce indirect light around just like baked lighting does.