There are eight partner Digimon you can get. They are Agumon, Patamon, Veemon, Guilmon, Renamon, Kumamon, Kotemon, and Monmon. After the first three you get, you will need to find the Ultimate level Digimon for the rookie that you want. DRI people are in various locations and they will tell you which Ultimate Digimon they want DNA from. Defeat the Ultimate level Digimon to get the DNA. Take it back to the DRI person that wants it and you will get that rookie. For example, talk to DRI person in Tyranno Valley. He wants DNA from MetalGreymon. Go to Bulk Swamp to find MetalGreymon. Fight and defeat him to get the DNA. Take it back to the DRI person and he will put Agumon in your team. Go to Digimon Lab or see Piximon to switch to Agumon.