How to get the correct angle when doing a visual appoach before landing?
PAPI

An airport might have Precision Approach Path Indicator (PAPI) lights set up. I believe these might be what you are thinking of. They are discussed in Chapter 2 of the AIM.

Here are two pictures to show them at night.
  



The first, shows a plane that is on the correct glide path. Firstly, they are normally installed to the left of the runway, and according to the AIM, red should be on the right, so, I believe the picture has been flipped, however you could still use them as is.

If you are right on the correct glide scope for that runway, you would see two white and two red. .2 degrees high would show three white and one red, and .5 degrees high, or higher, four white.

Conversly, .2 degrees low will show three red and one white, and .5 degrees low or lower will show all four red.

Red and White is Just Right. All Red, You're Dead.



VASI  

There might also be VASI, or Visual Approach Slope Indicator, lights. These are in many different configurations. They are also discussed in the AIM at the same link I posted above. An example of a two bar VASI light setup is shown below.


The VASI lights are the two sets of two lights next to the runway (just above the row of green lights. They are set up so that white over white is too high, red and white both showing, on glidepath, and red over red is too low.
The mnemonic for those is: red over white, alright; red over red, you're dead.

For more information, there is the AIM Page, the PAPI Wikipedia Page, the VASI Wiki Page, plus there are many other pages which are useful. If you search (using Google or BING, for example) for "runway lights papi" or "runway lights vasi" you'll get many hits.



One last comment here. The correct glide path for an airport is not always the same. If there is/are an obstacle, the glide path might be higher/steeper. If the runway is lighted, using PAPI and/or VASI lights, use them! They will keep you on the correct glidepath for whatever runway you're trying to use. 

Good Luck and Safe Flying!