This question is best answered by asking another question: where do you live?

If you live in the Northen hemisphere, then your  solar panels should face south. Likewise if you live in the Southern hemisphere, then point your solar panels north.

This is because in winter, the sun in the Northen hemisphere will angle from the south. (Because of the earth's tilt on its axis.) This means you will catch the winter sun, which is very important for solar power.

This depends on where you live in the world. The closer you live to the equator, the more horizontal your solar panels will be. This is because the sun tilts less in winter.

The further north or south you are from the equator, the steeper angle your solar panels need to be at.  Fortunately, there's an easy way to discover the optimum angle for your panels.

If you live at 34 degrees latitude, the best angle for your solar panels is 34 degrees off horizontal. If you live at 25 degrees latitude, you want to angle your panels at 25 degrees.

You can have your solar panels on a ratchet. This allows you to change if for summer and winter. This will make them more efficient, as the sun's angle changes throughout the year.

As a good rule of thumb though, angle your solar panels at your latitude angle. You can actually go fifteen degrees either side of this optimum angle without too much loss of efficiency. This is particularly useful to know if your roof slope is too steep or not steep enough for your optimum angle.