When a customer from another country such as canada uses the checkout it charges them in CAD i only want it to charge in USD can u pls help thx

Method 1:
In admin console,
System --> Static Tables --> Countries
Locate Canada. You may need to use the search at top of page.
Once located, click Edit Country.
In country details, toggle 'Default currency' to 'American Dollar'.
Save. Repeat for other currencies as needed.
 
In admin console,
System --> Static Tables --> Currencies.
In the list of currencies,
Select Edit to the right of Canadian Dollar.
In Show for User, uncheck both of the following:
(user can choose this currency in which prices will be shown)
(all prices shown by default in this currency)
 
If you want to ensure Canadian dollar never comes up as a possibility continue with:
2nd [permanent and additional] method:
In admin console,
System --> Static Tables --> Currencies.
In the list of currencies,
Select Edit to the right of Canadian Dollar.
In the Canadian Currency details page,
Scroll down --> Delete.
This will remove Canadian currency as an available currency for currency conversion.
 
While in the Currencies table, ensure that for American Dollar, you see 'Yes' under the column titled 'default'.
If not the default, click 'edit'.
In edit window for American dollar,
In 'Show for User', toggle 'x' for '(all prices shown by default in this currency)'
Country codes that do not have a match in currencies table will use 'default currency'.