We can't if it is too big.

Not too much. But that doesn't mean we shouldn't try. An asteroid strike is inevitable sooner or later. At our current level of technology, we could place several missile platforms in high orbit to launch multi-megaton nuclear warheads at incoming asteroids to either shove them off course or cause them to break apart into smaller peices. Of course, nukes in orbit is a political hot-potato.

We install a gigantic steering wheel at the North Pole, and at the last minute, make a hard right. When the danger is passed, we line back up in orbit and put everything back in the cupboards.

make a death star and blast them

Probably arrange a simultaneous nuke launch at the asteroid.... We already have ICBM's that are deployed into orbit for precision striking so it wouldn't be difficult to launch one at an asteroid

Depends when we find it and the size. Firstly early detection is key. Everything else is dependent on it. Blasting a large asteroid apart into smaller pieces is not a good idea. They will mostly continue to follow the orbit of their centre of mass towards the Earth. Then you will get clobbered by several smaller asteroids rather than one big one. This could end up being worse than one big impact. Not wise. However for a smaller object you may be able to smash the object into small enough pieces so that they don't make it through to the surface. Dangerous though as you would maybe end up tracking hundreds of objects instead of one which is a great deal harder. If you have time the best bet is to somehow deflect it. Explosives? Maybe but not ideal. The best way would be to somehow attach thrusters to the object and push it slowly but surely off course. But that would be a hell of an operation to pull off. If it is too close then there ain't much you can do except bunker down and hope for the best. There are a few other options but these are the most likely to work in my humble opinion. Kinetic Impact looks an interesting concept but I don't know a great deal about it. Of course what you really do is send Bruce Willis and a group of roughnecks to fly the space shuttle like it is the Millennium Falcon and then blow that son of a bitch to kingdom come! That'll work I am sure...

We could start by practicing the technique of having our school children hide under their desks, so when a big one does come at us out-of-control, they will know what to do. This was the mentality used when I went to grade school in Nebraska about 50 years ago. If the school had been hit by a tornado, that "drill" would not have helped us one bit. If an asteroid of significant size hits the earth, it's a new Ice Age and we're all screwed! There would be no place to hide. Otherwise, the above answers might work. God help us all!

With plenty of quarters!

Current scientific thought is that the best way to deflect an asteroid is to provide some nearby mass. Mass, even relatively small mass, will very slightly deflect an asteroid through gravitational attraction. This deflection is of course very tiny, but given the astronomical distances involved, even a microscopic deflection in course is enough to turn a hit into a wide miss. This relies on early detection and the ability to transport mass quickly to the vicinity of the asteroid, so it requires either luck or a much more comprehensive system of detection than we have today.