Goal setting is first step towards having successful and happy life. Success and happiness both are interlinked as successful actions taken towards completing your goal makes you happy. Important points to consider about setting goals
Make realistic goals: Your goal should be realistic enough to be realized. If you have failed class X but still you can make to to IIT/IAS as history is filled with such examples but at the age of 50 you can't break record of Sachin Tendulkar. If you chase some nun-realistic goals then at end un-fullfilled goal leads to frustration.
Make Long term goals: Make goals that in 1-20 years you want yourself to be doing something.
Make short term goals: Once you have set your long term goal, break the long term goal in short term goals. Success in short term goals motivates you to reach your long term goal.
What happens if you don't set your goal
Your life goes in dis-array: instead on guiding your life in particular direction you might be roaming around a point.
A person is happy once they attain their goal, if you don't have any then you will keep complaining about things whatever you get, as you never had any preference of anything. So whatever you get you complain about it.
Example: Suppose for whatever reasons you set your goal that you want to go from New Delhi to Mumbai. This goal might be part of your larger goal. Now we divide the goal in smaller goals (planning for attaining the goal) i.e I have to pack my luggage, find a taxi to reach railway station , take ticket and sit on the train, reach to Mumbai. If you don't plan and set smaller goals then you might be wondering in streets of New Delhi only for your whole life and never reach to Mumbai.
Set your goals and take control of your life and stop wandering aimlessly.