Scripting writing conventions are basically the standard way to format, and layout your script. I didn't do scriptwriting, so you may need to crosscheck this with a teacher. However, it should get you started.
Double Spacing, 12 size font.
Then formatting is as:
ACT I
Interior of Bill's Apartment. A lamp stands in the corner P. A large lime green couch takes up most of the middle of the room.
BILL
Hey, what happen to you the other night?
TED
Well... I was going to go to the shops. However, on the way I tripped down a flight of stairs. As I peered up towards the ceiling, a wave of relief flooded through my body.
BILL
Wow...
[enter Mary, OP]
MARY
I just left the... [A lamp falls down]
Centre formatting. Names are in capitals. Directions in brackets. Technical descriptions in Italics. Stage directions use upstage, downstage, P (Prompt), and OP (Opposite Prompt), but don't overuse them.