Hey, I started writing a screenplay today based off a true story and was wondering how you do it. Like, do you keep the everything the same or do you change some things?

Change how ever much you want or however little you want.  Even if it's based on a true story, it's your screenplay, and you don't want to ruin it just to get every detail right.
Look at Amadeus.  It was based on a real guy, but for dramatic effect turned him into a complete bawd.  This added a lot to the movie, but has little (not none, but little) evidence in reality.

Remember though, that if you change enough of it, you must also change the verb.  Instead of "based on a true story" write "inspired by a true story" or something like that.

The same rules apply to adaptation from a fictional source.

Hope that helps.

The TV Series Titus is based off Chris Titus' life and the way he tells it is that the basic premise is true but they added everything else to accommodate a television series.

You change what you must to get the desired effect.

It all depends on the desired effect you want the movie to have. Lots of movies are "loosely" based on true events, especially in horror - Psycho, Silence of the Lambs, Texas Chainsaw Massacre were all "based" on Ed Gein, though the actual stories never took place. Even movies like Hostel was a premise that  Eli Roth found may have been going on in Thailand. war movies also put false characters in real situations a lot of the time, Full Metal Jacket. A League of Their Own also put false characters in a real event. But there are also biographies like Monster and From Hell, where it's pretty accurate, but they still change things, or just show one side or angle of the story. So it depends on if you're doing a biography on somebody, in which case you'll want to make it pretty accurate to do the person justice, but know that you'll never get the dialogue word for word, but you'd want to do your research when you write about someone's life. Or are you just taking a real premise and building a false story and fictional characters on it, because then, you don't have to be exact. And you could change the names of real people as well if you just don't want it to be about the real people, and you want to change the story around.

Quoted from Old Time Wesley The TV Series Titus is based off Chris Titus' life and the way he tells it is that the basic premise is true but they added everything else to accommodate a television series. You change what you must to get the desired effect.

I loved 'Titus.' I wish they hadn't cancelled it.

I enjoy true events, but they usually aren't very dramatic or you can't find out very much of the truth behind them. Like with so many things, I prefer to find some bizarre truth and weave it into fiction. It gives you a good premise and total freedom to make the characters do what they need to do for the drama without worrying about the guy's momma suing you for slander.

Another example of true life inspiration is A Nightmare on Elm Street. Wes Craven had several newspaper articles about people who had died in their sleep, had complained about night terrors prior to their deaths, and even had pots of coffee in their bed rooms and had stayed awake for days on end before they died. This became the premise for his immortal tale whose villain immortalized a childhood bully.