The steering wheel vibrates at certain speeds, usually between 50 and 70 miles per hour?

This is usually caused by a front wheel out of balance. If you feel the vibration only in your seat, it's probably a rear wheel. In either case it pays to have all 4 wheels balanced at the same time. You will be surprised at the difference it will make in the way the car will ride.

My car drifts to the left or to the right when I let go of the steering wheel on a straight and level road?

First, check the air pressure in all your tyres, and then try driving it again. If your car drifts to the right in the right lane and to the left in the left lane, it is probably normal. Most roads are banked to allow for water runoff. Make sure both front tyres are the same size, type and brand. If not, take both front tyres and switch them side-to-side. If the car now drifts in the opposite direction, you've found your problem. If the problem is still there, you should have the wheel alignment checked.

When I drive down a straight road, the spokes of my steering wheel are not centered

This is also a wheel alignment problem, but one that is easy to correct by doing a wheel alignment. However, make sure the mechanic checks for worn or damaged parts. If the problem appeared suddenly, possibly after you hit a pot-hole or curb or something like that, then you've most likely have sustained some damage which should be checked as soon as possible to avoid serious tyre wear.

There is a humming noise coming from my tyres on a smooth road?

Possibly you haven't rotated your tires in a while, check the rear tires. Run your hand around the edge of the tread, first in one direction, then the other (Note: Be careful not to injure yourself on debris or exposed steel belts on the tire). You are looking for a "saw-tooth" wear pattern between the tread blocks. If the tread is smooth in one direction but jagged in the other direction, you may have found some of the noise. Try rotating the tires.