Can I connect 2 external monitors to my 12" Retina MacBook (early-2015)?
This MacBook can simultaneously support the internal display as well as up to 3840x2160 at 30 Hz or up to 4096x2160 at 24Hz on one external display. 

In other words, connecting two external Thunderbolt Displays is not officially supported by Apple. The two displays are the internal display and an external display.

If you're still interested in connecting one external display, since this MacBook has a single USB-C port and no traditional video port, you'll need an adapter to connect the display. Apple provides support for both HDMI and VGA via the following respective adapters:


the Apple USB-C Digital AV Multiport Adapter for HDMI monitors, and also allows you to connect a standard USB device and a USB-C charging cable.
the Apple USB-C VGA Multiport Adapter for VGA monitors. Like the HDMI adapter, this also allows you to connect a standard USB device and a USB-C charging cable.


Once you have the appropriate adapter, it's just a case of connecting the cable between it and your monitor.

Of course, you may choose to purchase 3rd party adapters instead of the Apple branded ones.