There are direct shuttle bus from Beijing airport to Tianjin yansha Bus Station. If you want to go by train, high speed train I guess, you should go to Beijing SOuth Railway Station. It takes one hour to the railway station and another half a hour by the speed train. The cost is around 100yuan in total by train. It is a bit complicated to Beijing South Railway Station from the airport. I have several modes for your reference:
airport express train to Dongzhimen - Subway Line 2 to Yonghegong then Line 5 to Liujiayao Qiaoxi. Take Bus 652 to Beijing SOuth Bus Station or you can take taxi from Liujiayao to the train station. Total fare: 28yuan or 47yuan. It looks complicated, but it is the fatest way. Or you can take airport bus line to Fangzhuang then take taxi from Fangzhuang to south railway station. It costs about 32yuan and takes about 1.5 hours.