By and until can be easily confused.  They are used differently and after a little studying, you should have no problems using them .
"By" is used to mean "no later than."  It is used like this: by + (a time) to express this.

Some examples of BY are:

"Please be home by 6:00 pm so we can eat dinner on time." - This means that the latest you should be home is 6:00 pm.  This is a set deadline.

"Every day Sean must be at work by 8:30 am or he will miss the daily morning budget meeting." - Sean can get to work earlier, but he must not get to work after 8:30 am because that is when the daily meeting begins.

"I must hand in my essay on Hemingway by Friday." - I need to finish writing my essay and give it to the teacher on or before Friday. This is a set deadline.
"Until" is used to express how long a situation continues.

Some examples of UNTIL are:

"We have to stay at work until our boss comes back from the meeting." - This means that even if our boss returns after the workday should have ended, we must stay and wait for him.  If our usual work hours are 9:00 am - 5:00 pm, and he will return at 6:15 pm, we must stay in the office and wait for him.

"Grandma said she will stay until it stops raining so she doesn't get her hair wet." - Grandma doesn't want to have wet hair, so she will stay wherever she is and wait for the rain to stop.  She will not leave before that happens.

"I will wait until they get here before I start cooking." - I will not start cooking before they get here, so after they arrive I will start cooking.