Based on your comment I'll assume your goal is to have a bipedal alien with a solid turtle shell.
The first thing to become clear about is the grade of protection the shell should offer. If protection is of little concern, you should go with the flexible armadillo-type proposed by Willk. Ninja Turtles also come to mind. Their shell protects only the back, but not the front and extrminies and is actually more of a hindrance than advantage.
If these aliens have to survive physical attacks from predators and should be bipedal, they either need a solid shell in the front as well or lay down on their vulnerable bellies when attacked.
Depending on the wheight of the shell, they might need strong limbs like those of elephants or rhinoceroses. If they cannot draw their legs into the shell for protection, they need extra shells on them (much like a knight in plate armor). If they're supposed to draw them in, you might need a rather big shell that's half empty when all appendages are extended and probably more joints than turtles have to move a pillar-like leg into a solid shell. In any case, short legs seem to be most feasible.
Now to the lifestyle: how would they eat food and drink water? Bending down, reverting to quadrupedal style to drink some water just to stand back up seems rather cumbersome. They could have elephant-like trunks instead or big, nimble hands that can scoop up water. Either kind of appandage would have to be long enough to reach the ground with, which would favor a small, stocky torso rather than the elongated shape of turtles.
Since intelligent creatures tend to have large brains, the head might be rather big. It would be an advantage to keep the brain inside the shell at all times, so the alien might poke just its face out of the shell instead of the whole head. Keeping the problem of eating and drinking in mind, they might have their head-opening at the middle or bottom end of the shell instead of on top.
To sum it up, there are 2 major anatomical variations feasible:
The nimble tool user
short legs retractible into the shell
long, slender arms with nimble hands retractible into the shell
short, stocky shell with enough room for a big brain, internal organs and all appandages
only eyes and orifice can be extended from the shell, brain stays in the shell
uses tools to make up for the shortcommings of its slender arms
the primitive forager
shell with very pronounced dome. Brain and lungs positioned at the top of the dome
head and heavy organs at the bottom of the shell to keep its balance and reach the food
short, pillar-like legs with broad feet to keep its balance
either elephant-like trunk or very short arms