How to build tails?

can anyone help me on this...?

Uh, a flexi cylinder attachment with a furry alpha texture? Maybe a little llSetRot() script to get it to wag on command? And/or maybe llSetPrimitiveParams() to make it do other tricks?

It depends on what kind of tail you want. Fluffy or thin, multi prim or single, flexi or non.

In the past all tails were made with multiple prims, usually torii or spheres.

Flexiprims added a new option, and many designs. With the exception of lizard like tails and really crappy fluffy tails, flexiprim tails are always made with several prims, usually cylinders (cubes would just look weird). The shape of the tail is defined with alpha textures that hide unwanted parts of the prims.

Sculpted prims bring yet another option. Though at the moment they can't be flexible, a sculpty tail has benefits because practically any shape can be made with a single prim. The shape can also be changed simply by changing sculpt textures, as opposed to showing and hiding a bunch of prims, as torus/sphere tails do.

My advice is to buy a tail with a design you like, and pick it apart to see how it was made. That's how I learned the basics of building.

If you're making flexi tails and you have more than one prim you want to move together, make sure the prims' Z values (their Z size) are the same, and that their flexi values are the same. When you first put the tail on they may be out of sync, but when you start moving they line back up.