The correct sentence structure should follow the standard rules of English grammar, where "am planning" is the correct usage to indicate an ongoing action in the present. The other options either use incorrect verb forms or improper tense.
Step 1: Correct usage of verbs
The phrase "I am planning" is the present continuous tense, which is used to express an action that is happening at the moment or is planned for the near future.
Step 2: Conclusion
Thus, the correct sentence is: "I am planning to attend the wedding party tonight."
Final Answer: I am planning to attend the wedding party tonight.