What will happen if there's already a visa available but the petition is not yet approved?

Hopefully it will be approved before the priority date is current but you can always try to follow up. If you do not get your visa and enter the US before you turn 21, you will automatically fall into the F-2B category and will need to wait until the priority date is current for that before you can get a visa.

It is a bit strange that your mother's I-130 petition has not been approved yet. You should call the National Service Center at the number that should have been provided in the e-mail that you received. Prepare to wait quite a while until you get to talk to a service representative. Have your filing receipt number ready. And tell the representative that your case is "outside the normal processing time" (right now, USCIS processes the I-130 petitions that were filed at the end of September 2011, so yours should have been decided about 4 months ago). You need not worry about aging out of the priority category: there is a federal law, Child Status Protection Act, that "froze" your age on the date your mother filed the I-130 petition; until a decision on her petition is made, USCIS treats you as if you do not get a day older. So, let's say, USCIS approves your mother's petition exactly 2 years and 1 month after the petition was filed; and the visa becomes available to you when you are 21 years and 7 months old - under Child Status Protection Act, you age will be counted as 19 years and 6 months, and you will remain in the F2a category.

Its still one year away.

May 2011 is your priority date if the I-130 petition was received by USCIS then. The visa bulletin would need to show a date of May 2011 or later for a visa to be available to you. June 2010 means you have at least a year to wait.