What level should I be to kill Alduin

Most of the story, including Alduin, scales very well with the player. You should not have a lot of trouble with any of the story encounters at any level, unless Bethesda meant for that particular part of the story to be hard (like the part leading to the final encounter).

One thing to keep in mind though, is that while enemy stats scale with the player's level in a way, things still can be made easier with equipment and spells. In other words, a character with top-tier equipment and lots of spells will have an easier job killing Alduin at level 30 than a character with cloth armor and no spells at level 60.

That particular part of the game where you are now shouldn't prove any trouble, because:

When you go to Whiterun, all you gotta do is pull a lever to trap a dragon. After that, you can enjoy a little ride. :)

I think you should be fine – I was level 32 when I did it. The best thing to do is save excessively, try it, if you fail, then grind to improve some skills.

Trial and error is the best option – besides, you may find that on your first try you succeed.