Add the lead wires from the controller to the track in at least 2-4 locations around the layout. You can solder the lead wires to the Atlas rail joiners then install or you can purchase rail joiners with the lead wires already soldered.
Welcome to the forums and to model railroading.
If I remember correctly, the Bachmann track comes with a sort of plug in arrangement on both ends. If you are not comfortable soldering yet, you could cut the track end plug off the wire and use wire nuts to connect to wires going to the track. Atlas does make a rail joiner with wire attached, as mentioned above. If the power pack end does not have a plug, but the two wires just connect, use regular wires coming from the rail joiners. Connect them together (wire nuts will work, but are not the preferred method, though I have never had any problems.) It is a good idea to have several connections around the layout. Are you going to use flex track or sectional? Many folks recommend a connection at least every 6', some closer, especially if you are using sectional track. One thing to be sure is that your joiners are tight, if one gets damaged, it will make a poor connection. Get a package of new ones to use as replacements.
You can use your EZ track with the Atlas Code 100 if you don't want to waste it. You would have to get cork or other roadbed to bring it up to the same height, cut a notch to go around the connecting end of the EZ roadbed and add your Atlas from there. You may need a thin shim at the joint between the two if the height isn't the same. I thinkk the rail joiners are just about the same size.