Draw a Ti atom with 3 electrons in it's outer shell. Add the 4 Cl atoms showing the electron which they donate and share in the covalent bond. You should encounter a problem in that there's 4 Cl atoms but only 3 outer shell electrons, so you can only form 3 Cl-Ti bonds right?
Now add the 3 extra electrons.
One of these 3 extra electrons can now form the final (4th) Cl-Ti bond, thus we are left with a lone pair as well.
A common misconception is that you can only have 8 electrons in the outer shell; that's GCSE. We now consider the orbitals and Ti's vacant orbital (outer orbital) is a d orbital which can hold 10 electrons.
Are you sure it's TiCl4 (-3) though? I'm not sure that exists, lol.