I had this issue when I first started. The greatest tip I was given which helped massively is write 16bars of your track, these 16bars should include all the main parts of your tune all the drums, bass, lead sounds etc. EQ and process these once you've got these 16bars working duplicate them back to create your desired length intro and remove certain tracks this should instantly give you the bare bones of your intro and help you make decisions on sounds that will compliment it, pads, atmos etc.