How do I get a button to open another activity?

A. Make sure your other activity is declared in manifest:

<activity
    android:name="MyOtherActivity"
    android:label="@string/app_name">
</activity>

All activities must be declared in manifest, even if they do not have an intent filter assigned to them.

B. In your MainActivity do something like this:

Button btn = (Button)findViewById(R.id.open_activity_button);    

btn.setOnClickListener(new View.OnClickListener() {         
        @Override
        public void onClick(View v) {
            startActivity(new Intent(MainActivity.this, MyOtherActivity.class));
        }
});