Aces

Aces represent the energy of each suit in its purest and most concentrated form.
They also represent beginnings, ideas, commencement, opportunity, a fresh start, inspiration, a gift. They are the root from which the cards that follow them grow.

Applying the above to what we have covered in the suits gives you the meaning of each ace.

Ace of Wands - New energy, a burst of creativity, beginning a self improvement program, the start of a spiritual quest.
Ace of Cups - A new relationship ( romantic or otherwise), awakening intuition or psychic powers, the start of happier times.
Ace of Swords - New ideas, a new intellectual interest or hobby, strength, power.
Ace of Pentacles - Beginnings on the physical plane, a new job, a new home, the start of a period of prosperity.

Timing

Many readers use Aces as timing cards.

If a question of timing comes up during a reading, write it down and put it aside. After the main reading is done, have the querent re-shuffle and cut the deck, then deal cards face up until you get an Ace. The suit of the Ace will indicate the time that the event is most likely to occur.
Using the seasons from the message on suits we have:
Wands - Spring
Cups - Summer
Swords - Fall
Pentacles - Winter

Yes/No

A method for answering yes/no questions uses Aces as well.
Shuffle and cut the deck while concentrating on the question.
Turn one stack over periodically while shuffling in order to get reversals. Deal the cards until you get an Ace, then stop.
Deal a new pile until you get another Ace. Deal one last time until you get a third Ace.
The direction of the Aces will give you your answer.
If two or more are upright, the answer is yes.
If two or more are reversed, the answer is no.

Personally, I rarely use Tarot to answer yes/no questions. I believe that the answer will be correct for the conditions at that time, but as conditions change, the answer may change as well.
I prefer to ask questions such as "what can I do to enhance my chances of...." or "What am I doing that is preventing me from..." . This type of question gives me some control over events rather than allowing things to just happen to me.