Once you've opened up weapons.xtbl (open with notepad or notepad++ if you didn't know already) then hit CTRL+F, type in
tank01 (make sure search direction is
down) and hit enter/find next, keep clicking until you find an entry named <weapon>sp_tank01_w</weapon>
Once you've found that, go to the Find box again (make sure the search direction is still down) and this time punch in the word
damage, it should go down to an entry called <npc_damage>1000 or some number</npc_damage> and right below that is <player_damage>100 or some number</player_damage>.
However, however, the sp_tank01_w weapon includes the main cannon
and the coaxial machine gun, so there's gonna be a section that includes the values for that machine gun, so make sure the player_damage value is the
higher number - this is the value for the main cannon, the lower number is therefore the machine gun).
npc_damage is somewhat self-explanatory, but seeing as you're new, I'll tell you anyway, it's the value that determines how much damage other characters in the game do to you with that weapon.
I believe at the bottom of the weapons.xtbl file there is a whole section of the value names and a description of what they do, I can't remember though.
EDIT: I also highly recommend getting
Notepad++, it's basically the Windows Notepad on steroids. It also lets you download plugins through the toolbar that make editing so much easier, in this case, the XML tools plugin where you can see the stuff you want to edit in a WAY cleaner format if it hasn't already been sorted yet.