an octahedral molecule will have 6 electron pairs around the central atom. And all of them will be bond pairs. Now, there will have to be 6 Fluorine atoms. You should look for a central atom which can exhibit valency 6. Group 6 elements, such as sulphur. The group 6 element should also have vacant and accessible d orbitals. Oxygen will not fit.
Trigonal bipyramidal molecules have 5 electron pairs. All of them are bond pairs. If one of them were to be lone pairs, the shape would not be trigonal bipyramidal. Following the same logic as above, there should be 5 fluorine atoms. You should search for a Group 5 element, having vacant and accessible d orbitals(not nitrogen). Phosphorous is ok.
T shaped molecules have 5 electron pairs, since this is the only combination, where 3 are bond pairs and 2 are lone pairs. It resembles the trigonal bipyramidal molecule. The two lone pairs will minimise their repulsions by being at 120 degrees mutually, and be on the flat trigonal part. There will be 3 fluorine atoms across the three bond pairs.
Hence the central atom should have 7 outermost electrons (3 for bond pairs and 4 for the 2 lone pairs). A group 7 element, will be ok, such as iodine or chlorine.
this is the best way i found to draw structures. First determine the number of B.P. and L.P., and then find the central atom.
hope it helps