passwords in flash are not safe. there's apps around that can crack an swf, and get all the variables and stuff out of it.
However, i guess what you are asking is how to use input text. Make a text, under text options, select "input text". There's a box "Variable". let's call it "age". This textbox is a variable, its name is age, and the value is whatever the user puts in. Let's presume we want to ask the age to decide if some-one can enter the site. Some-one fills in: 12. Below the textfield there's a button that says: ENTER. here's the code of the button:
on(release){
if (age<=17) {
message="sorry, you're not old enough";
}else{
gotoAndStop("site"

;
}
}
"message" is also a textfield. it's a dynamic textfield, here, the "Variable" field says the word: message
whatever text you put in the field, if a user fills in 12 in the input age field, the message textfield will display : sorry, you're not old enough.
that's it. try and play with it.