How often should I weight train to get the very best benefits?

4-5 days/week. To increase muscle mass, strength training 4-5 days/week with 2 recuperative days is recommended. A diet high in protein should be sufficient nutrition, but if you are not sure about your protein intake, then a protein supplement should help.
3x per week. Lifting weight every other day allows the muscles to heal before stressing them again lifting weight.