Why did you change the thermostat?  Were you having problems before
you changed it??

What kind of car and engine is involved?

Well, first off if you just did something and it has caused a problem... I'm
sure you can figure out what comes next.

Let's start though, with a bit more information.  How about -

What kind of car are you talking about?
What size engine in the car?
What caused you to change the thermostat in the first place?
Are you certain you installed the thermostat right side up and not upside
down?
Where is it leaking antifreeze from?

When you reply please take the time to use punctuation and reasonably
complete sentences.  It's too much of a hassle trying to figure out what
people are saying when they don't use punctuation and you're the one looking
for help, so make it easier on people here to give you that help.