First, the variable part: suppose you define a variable var value = 0. To increase it by 10, you can write value = value + 10, but in JavaScript this can be shorten to:
value += 10
The same way, to decrease it, just write value -= 10.
To call the function, you write onClick="someFunction()" (not the best practice), and then you define the function:
function someFunction(){
value += 10
};
This is the working fiddle: https://jsfiddle.net/gerardofurtado/6qh1yhsj/
If you want to see how to do the same thing without the onClick="someFunction()" part, here is a fiddle: https://jsfiddle.net/gerardofurtado/wff8mph3/
PS: I see that in your code you wrote var team2 == 0. In JavaScript, two equal signs make a comparison operator: http://www.w3schools.com/js/js_comparisons.asp
PS2: In JavaScript, you have to mind the scope. Once you defined the var team1, you can change it inside the function just by writing team1. But if you do as you did:
function someFunction(){
var team1 = team1 + 45
};
This team1 is not the same previous variable team1 defined outside the function. It's a different variable. And, as the team1 to the right of the equal sign is not defined, this will return a NaN (Not-A-Number).
team1 = team1be Team1? With a capital "T" Also I don't see anything callingclicked()writeText()assuming you want the to be called when those buttons are clicked...onClick="readText(this.form);writeText();to your button inputs. Also where is thereadTextfunction? And why are you passingbutton1into thewriteTextfunction but never using it? Can you display all relevant source code.