At the simpler level, it is very easy to make things destructible by any and all attacks, from knife up to artillery.  It is more involved to make say a wall destructible only by dynamite.

We're going to start at the simple end and make some fences you can shoot to bits with bullets.

Run Radiant, open the map, and make a fence post by drawing a box as shown.

Ctrl+tab so we can control the height of the fence post and ensure it stands on the ground.  Caulk it of course too.  Press ESC.

Apply some wood texture to the 5 visible faces.

Select the fence brush, then in 2D, right-click and select Make Detail.

Select it again, and again right-click, but this time choose func/func_explosive.  Press N.  Close the window and press N again.

Tick UseShader box.  Enter key "health" and value "100" and press return.  A bullet does around 15 damage.  Grenades do 200, and panzers do 400.

Enter key "type" and value "wood" and press return.  Close entities window.

Press 8 and duplicate the fence post, then move it into line as shown below.  Duplicate it again to make a third one in line.

Press 7, and duplicate to make another fence post which we'll do something different with.

Right click in 2D and select Ungroup Entity.  This returns the brush to an ordinary thing instead of an explosive one.

Duplicate the post.  Press ESC.

Use grid scale 3 to create the linking fence plank as shown (this time allow wood texture to remain on this - do not caulk).

Ctrl+tab to see how tall the brush is, and make it fence bar size.  Right-click and Make Detail the bar.  Select the brush again.

Then select the upright posts so that you have all 3 brushes selected.

Then in 2D, right click and select func/func_explosive.  Again Press N.  Tick UseShader.  Put the cursor on the word "wood" in the value box and press return.  This is a quick way to set the same value as used earlier.  Also put "health" in the key and say "150" in the value and press return.  Press ESC.

We now have 3 standalone posts that can be destroyed individually - and a fence that when destroyed, all 3 components go up together.

This is to illustrate that you can make a single explosive entity out of any collection of brushes (but not of models) and that you mustn't create something that might have its upright posts destroyed, but leave the crossbar intact floating in the air!  This why you'll often notice that crossbars can be destroyed, but their posts cannot (eg Fueldump near the tank start).

Save, compile and go shoot some planks :)