Ok here we go....

Crates-

Make a brush, create a crate. I assume you can make a box that has a crate texture.

Once the crate is made, right click the crate while it is selected (highlighted red), then select func>crate. Now it is a crate. press "n" with your newly made crate selected. See that nox that says nonstackable? if you don't check this any crate stacked on top of this crate will fall if the one below it is destroyed. Most of the time you want thisSometiems you don't want this. Let's say you boarded up a window and want the boards breakable. Just highlight the boards, rightclic and give it func>crate. But this time press 'n' for the properties window and check the nonstackable box in the window. Now the boards won't fall above it so it will look more realistic.
You can give it any amount if health by putting these key/value in the properties window after hitting 'n'.
KEY: health
VALUE: (whatever, let's say 100)
(there is a default setting for health so you don't have to.)



BARRELS:

Make a brush about 2/3 the size of a player. Up at the top click where it says brush and select either a 9 sided brush or and arbirary sided brush of whatever amount of sides to the barrel you want. I use 12 or 13. (I don't make a cylinder because a bunch of cylinders will draw on the cpu resources.)
Find the appropriate barrel texture (red for gas) for the sides and top and bottom.

Now select the brush and right click and give it func>barrel
Now you can hit 'n' to get the properties and in the key/value part.
key:barreltype
value:gas

Now you can change gas to oil or water or empty.
The gas ones blow up everything. Oil ones are affected by explosions.
(BTW if you want a crate yo explode just make it a fuckin barrel with func>barrel)
Hope that helps.
-GENERAL COBRA