There is a fairly convenient bus service that runs between Tsukuba and Narita airport, but it doesn’t run in the early morning or late at night. For example, the earliest bus leaves Tsukuba at 6am and arrives at Narita Airport at around 7:45am. If you need to get to the airport even earlier, you have basically two options.
Hotel Near the Airport
If you have to be at the airport by 7:30am, for example, I think your best option is to go to Narita by bus or train the night before and stay in a hotel near the airport, like the Narita Sky Court Hotel. (I have never stayed at the hotel, but it seems to be one of the cheaper options.) Or, if you don’t mind having less luxury, you could try a guest house like Azure Guest House.
Another option is to go by train early in the morning. Take the Tsukuba Express from Tsukuba to Kita Senju (5:28am to 6:01am). Take the Joban line from Kita Senju to Nippori (6:15am to 6:23am). Then take the Keisei Skyliner from Nippori to Narita Airport (6:35am to 7:29am). The cost will be 3070 yen per adult and the total trip will take about 2 hours. These times are for weekdays. The schedule will be different for holidays, and also the schedules change on a regular basis, so please look up the schedule for the actual day that you need to travel. There are plenty of Japanese sites that will let you search for train schedules, but if you can’t read Japanese, try Jorudan.