Help with sentence construction
The interpretation of 我本來 and 衹是 are fine.


抱着 (literally to hug, but should be interpreted as to grasp onto)
試一試的想法 (a thought of just trying)
參加比賽的 (to participate in a competition)



  我本來衹是抱着試一試的想法去參加比賽的
  
  I originally participated in the competition holding on to the thought of just giving it a go