My fortress is overflowing with food barrels, but there is very little drink left. I do have seeds for brewing more, but cannot, since there are no more barrels. There is no wood available for making more. What can I do?
To set a number of reserved barrels, hit p to bring up the stockpiles menu. On the right, about half-way down, you'll see the number of (fortress-wide) reserved barrels. The keys to change the number of reserved barrels are shown after the number of barrels currently reserved, use * to increase the number, / to decrease the number. I personally recommend reserving at least 20.
If your number of empty barrels drops below that, barrels will not be used for storing dry goods (seed bags, plants, meat), they'll only be used for jobs that require barrels, such as brewing.
If you're running low on barrels for brewing, the best solution is usually to make rock pots, by enabling stonecrafting for one of your dwarves, and selecting first
make rock pot at a craftsdwarf's workshop.
Create a stockpile of food. Set it to only accept drinks. Then, set it so its reserved barrels is half the stockpile (ie, 24 space stockpile, then set the reserved barrels to 12). This means that 12 barrels will always be kept in the stockpile, so they will be available to the brewer for making drinks.
As for making them, you can make barrels from wood (using carpenter's shop) and metal (using a forge). You can make large pots from stone (using a craftsdwarf shop).