I don't know where you live, so I don't know if there is a color code used like there is here, in Massachusetts. Typically, white is neutral. Neutral and ground are not really interchangeable terms to electricians - but often are to homeowners. Read this until you understand:
1) If you have two separate bars in your panel, and one has only white and / or gray insulated wires connected and the other has only bare or green or green and yellow striped insulated wires connected, you must connect the white wire to the bar with the white / gray wires. Do NOT connect it to the bar with bare / green insulated wires.
2) If you have one or more bars in the panel, and both have a mixture of white / gray and bare / green wires, and the bars are screwed directly into the panel back - or have a jumper connected between them, you should be able to connect the white wire to any available terminal on the bar(s).
Colored insulated wire (not green, white or gray) usually means it is not neutral or ground. In an electrical panel, most times if it is not neutral or ground, it is "hot". If there are two colored (hot) wires and a single white or gray (neutral), the white goes to neutral bar and the hot wires go on adjacent circuit breaker terminals - in this case, a 2 pole circuit breaker.
Your gas or oil fired furnace likely operates on 120 volts (hence the white wire for neutral), while the AC compressor and blower is probably 240 volts (hence the need for the second colored wire). You may need to connect these two hots to one "2 pole circuit breaker" for the AC to work.
A typical 2 pole circuit breaker has two handles tied together - and internal connections.
Look at the wires to get an idea as two what size breaker they need to be connect to - #8 copper 40 amps, #10 copper 30 amps, #12 copper to 20 amps and finally, #14 copper to 15 amps. If you are unsure, and my background on the way things should normally be do not jog your memory, STOP. call an electrician and have it connected correctly and safely.