In SP 2010 and 2013 it is not possible to set a default value in lookup field in a NewForm from a List by default, but here are a few steps to get this issue working:
1. Open SP 2013 Designer and select the list which you want to set to a default value for. Click on 'Forms' and right click on NewForms.aspx and then advancedMode
2. After that start the SP Form regular with e.g. Internet Explorer and press F12 for developer console. Select the DropDown lookup which you want to set to a default value and get the marked #Title for the option field:
3. Edit the the aspx Code of the NewForm.aspx as it is shown in 1. and put the following Code after Content Placeholder Main. Be sure Jquery libs are referenced on a local folder in SP. DON'T USE the #ID tag it IS NOT WORKING !!!