Does anyone here know how I can get the eyebrows and hair on a figure to look more realistic? And how can I change the texture of the skin?

Which models? Which hair? Changing eyebrows is a matter of either repainting them or obtaining better textures, or using better quality transmaps. Same with hair. Skin can be improved by using higher res, higher quality textures, and/or adding bump maps.

Are you using any texture, or just the flat skin colour? You make changes by selecting your figure and going to Render>Materials, or pressing Ctrl(Win)/Cmd(Mac)-U. What you change, and what you use, depends on the model as SamTherapy said.

I am using the P4 nude man figure. And I have just been changing the flat skin color.

The flat skin color never looks realistic. 1) Try searching free stuff for a desent texture and the MAT file to turn him All White. 2) Then work with the lights (check the tutorial section or download some free sets - Traveler has some EXCELLENT skin tone lights at RDNA). Lights play a big part in getting a good skin tone. 3) Also like SamTherapy says you need a good bump map; I usually play with them or create my own (grey-scales of the actual texture)