It is all a matter of taste really. There are a lot of nice packages that feature really good trees.

Look in the scenery section for some nice examples and experiment with them.

Gerrish Gray's trees package is very popular as are the ones made by Arno Gerretsen or Feng Zhu, of which the lattar is also included in Real Environment Pro by Tim Fuchs. There's also an addon for REP called 'Real Forest Environment' made by Vagner Dalbosco, which is also very nice.

Try a Google search if you can't find them.

Please be aware that installing tree textures change the look of your sim and if you do not backup your original textures it can be very hard to restore them.