Okay, so I was asked out 4 days ago.
Of course, I consented because I have liked this guy for a while.
But you see, I was dating someone who I did not know was his friend
The week before and I didn't have the strongest feelings for him.

So I was asked out, and we were really happy. He made me really happy.
Everything was super great until my ex told him lies about me, and my boyfriend
Dumped me over text.
It was so sudden. But I called and asked if I could see him in person, so
We just sat out on the grass in a schoolyard.
We talked occasionally, mostly me trying to explain the lies that my ex told him.
It was awkward. I just wanted to cry and beg for him back, but I didn't.
He had to leave, so I did too.
I really want him back. Any advice?

You sound really young.

If there are so many lies, how do you know who and what to believe?

If the guy you started seeing is so gullible to believe lies and not see that he is basically out for revenge you are probably better off without him. If you explained that he is lying and he can't buy it then it is time to move on.
Neither one is worth it.

It's a big red flag, when your guy listens to lies from an ex. It so sad he believed him over you.

Leave him alone, and let him think about what a boob he was, as its his loss.

He is the one with the problem, not you.