It's probably easier to break your challenge down into smaller questions.
Do you know how to build a hovercraft?
Do you know how to steer a hovercraft?
Do you know how to program a microcontroller?
Do you know how to detect a line?
Do you know how to turn line detection into steering commands?
Once you answer "yes" to all those questions, you have what you need. I suggest you start from the top, and work your way down.
I probably know less than you about questions 1 and 2.
For question 3, I'd suggest an Arduino Leonardo or Nano or similar.
For question 4, I'd suggest a Pololu QTR-8RC detector board, plus software on the Arduino.
For question 5, I'd suggest a PID algorithm.