its normal to be stiff when you just start out when someone puts a technique on you. eventually u learn to let go and go with the flow
It will seem like information overload and you've got 2 left feet when u just start. you can take it slow and learn where to put your hands and where to put your feet. pay close attention to what the sensei is doing and just follow the senior students.
don't forget to breathe when receiving technique and most importantly have fun!
There is no secret in Aikido, just training and more training
hope this helps