Your energy/tech level needs to be 3. So, you have a few options.
1: Make a Temple and upgrade it 2x
2: Build a Temple, upgrade it 1x, and Occupy a Reactor
3: Make a Temple and Occupy 2 Reactors
4: Occupy 3 Reactors
Now, having a Scarab isn't the easiest thing to do early in the game. But here's a good strategy I like to use that takes about 12-15 minutes.
***only works in 3v3***
1: Choose the Prophet
2: Build 4 Supply Pads
3: Send Ghost out to collect Resources
4: Build a Temple
5: Upgrade your Base
6: Build another Supply Pad
7: Send Prophet to attack an NPC Base
8: Build Shield Generator
9: Upgrade all Supply pads
10: Upgrade Temple
11: Upgrade Prophet Chair
12: Build another Base (#2)
13: Send Prophet to another NPC Base
14: Build 3 Supply Pads
15: Upgrade Base#2
16: Build 2 Supply Pads
17: Upgrade Prophet Chair
18: Upgrade Base#2
19: Build 2 Shield Generators
20: Upgrade all Supply Pads
21: Upgrade Temple
22: Build Another Base (#3)
23: Build 3 Shield Generators
24: Upgrade Base#3
25: Build 2 Airfields
26: Upgrade Base#3
27: Build 2 Airfields
28: Destroy 1 Supply Pad at Base#1
29: Build Shield Generator at Base#1
30: Upgrade Prophet Chair
31: Upgrade Reinforcements
32: Get Ghost destroyed
33: Lock all Bases
34: Build Scarab at Base#1
35: Build Scarab at Base#2
36: Build ×10 Engineers at Base#3
37: Upgrade Engineers 3x at Base#3
38: Set Base#3 Rally Point to Teleporter
39: Upgrade Prophet Beam 3x
40: Unlock all Bases
41: p0wN The Enemy
