Start by opening the Wireless Interface window in Winbox. You will see some wireless cards listed here, they might be disabled – to turn them on, click on the blue Enable button. Make sure that the interface is configured and the antennas are connected before you enable an interface.
To configure an interface, double-click it’s name, and the config window will appear. To set the device as an AP, choose “ap bridge” mode. You can also set other things, like the desired band, frequency, SSID (the AP identifier) and the security profile.
You probably want your AP to be secure, so you need to configure WPA2 security. Close the wireless setting window with OK if you are done, and move to the Security Profiles tab of the Wireless interface window. There, make a new profile with the Add button and set desired WPA2 settings. You can choose this new security profile back in the wireless Interface configuration.
To see if any stations are connected to your AP, go to the Registration Table tab in the Wireless Interface window.