What should you log?... Virtually everything, even though you are doing extract.
If you are making a starter, log the size if the starter. Otherwise log the yeast strain and number of packages or vials you use. The expiration date from the package/vial, in case you have problems.
Your steeping grains and how much of each.
Your water temp during steeping (I always added mine to the water and then brought it to temp and held it there for 15 to 20 minutes).
The volume of water that you are using to steep and boil can be the same, but make sure to log it.
The weight of the extract that you are using (remember not to add this to your kettle while the flame is on, so that you don't scorch it). What point that you add it to your water (if you add only part of it at the start and the balance later).
Your hops - type, amounts and time that they are added to your boil.
Any adjuncts - type, amounts and time they are added. (this could be spices, fruit, honey, etc,)
Total boil time - 45 mins, 60 mins, whatever the time from start of boil to flame out.
How you chilled your wort and the time it took to chill to pitching temperature.
How much top off water you needed.
Original Gravity or Starting Gravity (make sure that you have stirred your wort very well before taking the sample).
Time and date that you pitched your yeast and the temperature.
Apparent fermentation temperature (unless you have a temperature controlled chamber).
Observations from your brew day - what could have been done better, done before hand, done differently, etc.
Gravity readings after fermentation period - this can be quick or long, depending on the OG and type of beer and the amount and type of yeast you used. Taste the sample and make notes on the taste, color and gravity. Check again in 2 or 3 days to see if the Final Gravity is still the same (if different wait another 2 or 3 days and test again).
Bottling Day - Your Final Gravity reading, this can be done in your bottling bucket before you add your priming sugar. The type and amount of sugar you use to prime, the date and temperature that you store the bottles at.
Good luck with your first brew day! Just relax, pay attention to what you do, if you create a great beer, you want to be able to repeat it and your notes will help.