If I understand your question, you have trouble making the transition to a barre chord from a chord in the first position where a barre is not used.

Try arranging your fingers so that the index finger is left free, ready to form the barre when its needed. For instance, play a C chord leaving the E and A strings mute, and use only your pinky, ring and middle finger to form the chord. Your index finger can then hover over the strings, ready to drop into place for the F or G that is likely to follow. You can use this technique to speed up the change from an A or a D to a barred G.