Commands are entered in prefix notation, meaning the operators come first. For example, (+ 1 2) evaluates to 1 + 2 = 3.
Besides basic arithmetic, other operators include:
define - This is how you declare variables. Example: (define a 1). Now if you call a, the value is 1. The return value of a define expression itself is 0.
set! - This is how you update variables. Example: (set! a 2). If a were previously defined as something else, now if you call a, the value is 2. The return value of a set! expression itself is 0.
lambda - This is how you declare a function. Example: (lambda (x y) (+ x y)). This creates a function which takes parameters x and y and returns x+y. If you call a define on it with a name, you can then call the defined name to use the function. Otherwise you can just pass the parameters after, as such: ((lambda x (+ x 1)) 2). This would return 3.
"="" - The equality operator. Example: (= 1 2) tests if 1=2. If it does, it'll return #t, meaning true. If it's false (which it is), it'll return #f, meaning false. There's also <, >, <=, >= for testing other conditions.
if - The if operator for doing conditional statements. Example: (if (= 1 1) 1 2). Tests if 1=1, if it's true (which it is), it'll return 1, if it's false, it'll return 2.
quote - This can be shorthanded by a ' in front of an expression. It returns the expression as a literal. Example: (quote (1 2 3)) or '(1 2 3) return [1, 2, 3]. Use this when you want to use expressions in operations, like (set! a '(dog)) sets the variable a to "dog"
cons - For concatenation. Example: (cons 1 '(2 3)) concatenates 1 to the front of [2, 3], resulting in [1, 2, 3].
car - Returns the first value in a list. Example: (car '(1 2 3)) returns 1.
cdr - The opposite of car, returns everything but the first value in a list. Example: (cdr '(1 2 3)) returns [2, 3].
begin - If you want to get fancier, use this to chain expressions. Example: (begin (set! a 1) (set! b (+ a 2)) (* b a)) will perform a = 1; b = a + 2; b * a = 3