Why are needed?
Simply because you also need a deadline from your builder when the house is finished. And for sure, you ask for a delivery date when you go to your tailor. Plus, you ask for a price. No deal is closed without a price indication and a delivery date.
Simple tricks to make it better:
1. Read down what you have to do
2. Understand it
3. Split it in pieces.
4. Write down the pieces and think how much would you spend for each.
4.a. You have do in it and can look back in the history. Put the figures there.
4.b. This is the first time you have to do this. Split it in more pieces and go back to step 4.
4.c You cannot escape from step 4.b. Use your intuition. How complicated does it seems?
Same other task : put the same effort as for that, plus the learning effort
Twice as complicated: twice the effort plus the learning effort.
Five time as complicated than what you did so far: ask for another task.
5. Think a little about risks: what can happen to prevent you for completing the task. Put some effort there (not for all risk you identify, only for half of the big ones).
If the risks are to high, ask for another task or tell this to manager.
6. Sum it up and see if you can reach the deadline.
If this is not possible, then see what is the delta missing: can you speed up and delivery in time or is basically impossible and should tell this to the guy who requested this.