If you press "Ctrl+A" to select all (Make sure the objects are highlighted), then "Ctrl+J", all of the separate pieces of the model will become one single object. You must do this in "Object Mode".
Also you may want to press "R" then "90" then press Enter. To get your model to stand upright.
Also you may want to press "R" then "90" then press Enter. To get your model to stand upright.