There are two reasons for toning black and white photographs: to achieve greater longevity of the image by protecting or altering the silver in the emulsion of the print or to change the feeling of the photograph by modifying the print color.
Selenium, sepia and gold are the most popular types of toning used to enhance the archival permanence of the image. These toners also can alter the color of a print depending on the type of paper used, the type of developer used and the dilution of the toner. If altering print color is not desired, I would suggest discussing this with the very knowledgeable staff at Freestyle to determine what the best paper, developer and toner combination might be for your purposes.
If you want to change the mood of your photograph, there are toners available in a rainbow of colors for this purpose. This type of toning can be very creative. Selective areas can be toned. Multiple toners can be used on the same print. Certain types of toners can be reversed by re-emersing the print in paper developer, or a more delicate color shift can be achieved by greater dilution of the toner. Toning can be the most fun a darkroom printer can have in the daylight. Many times the power of an image has been revealed to me by experimenting in this manner.
There are several books currently in print on toning. Freestyle usually has titles in stock that are very helpful. If you want to buy a toner and experiment yourself there are a couple of rules to follow. Always wash your print thoroughly before any type of toning. Also it is a good idea to have separate trays and tongs for toning. If using multiple toners wash you print well in between toning baths. Save your reject prints for toning experiments. Using test strips can let you know how the toner effects the shadows and the highlights or if the toner darkens or lightens the print. The most important thing is to keep notes so that if you come upon a wonderful combination you have a good chance of repeating your success. Enjoy!