Welcome @Mary_Helen26
You’re in the right place. Lots of expertise.
We need a bit more info ie your medium. Soil? Coco? Tent size? Wattage?
If you don’t have lights, talk to @AutoGrowerMo he builds lights and knows his stuff.
#1 mistake is Overfeeding and Overwatering. Fresh soil doesn’t need feeding just yet. When feeding start with ½ strength nutrients and make sure there is good drainage. Watch pH. 5.8-6.2 depending on medium.
#2 mistake is Doing Too Much. Especially as a beginner! Stick with simple training your first grow. As you learn the plant you will add more techniques and know when to apply them.
Opinions vary. I usualy grow autos on 18/6 but have done 20/4. Heat is a factor in my tent.
Some people do 24 hours but honestly I think they need a dark period.
Pot size. Depends largely on strain. Big plants need big roots. Size of space, environment, plant count…
I tend to grow 3-4 small to medium plants in a 3x3 so I use 2 or 3 gal bags of coco. I used 3 and 5 gallon when I grew in soil.
I sprout seeds in starter cubes and when I see roots at the bottom I put them in their final pot. Autos are on a fixed schedule meaning they don’t have much time to recover from transplant.
Most techniques to maximize yield involve stress. There are LST (low stress tech) like bending and tying, and HST (high stress tech) like topping, pruning, etc.
#1 technique is healthy happy plants 
As a beginner I’d start with simple LST bending and tying if anything. …There’s also value in watching a plant grow without intervention.
Have fun! Drop by Live Stoners for fun and lively discussions