There are several ways to move an object to the center of the viewport, (The big Screen Area in Blender).
First, it is a good idea to determine, based on the modeling you are doing, where you would like the center of Origin for the model.
You can set the Center of Origin by right-clicking on the model object, and selected the center of origin. There, several options will appear. I usually set my to the Center of Mass. It is just the way I work, and the choice is yours.
From there, to move the object to the center of the view port, or to perhaps the 3d Cursor Location, select the object in the viewport, and press Shift-“S”… There, a menu will appear and you can choose where to place your model, for not, select 3d Cursor, since it is probably already in the middle of the viewport.