I am new as well to brewing and BIAB. I've done both kits and recipes with beersmith.
I stressed for a while over this, and did tons of math and checking in multiple software, along with my own mass balance calculations (I'm an engineer)....all of this was in the end not really needed. If you have a refractometer, you can be simple and precise, and have fun. If not, you're probably still fine.
If using a 5 gal kit:
Choose a simple recipe in the 1.04-1.06 SG range. Ask to double grind for BIAB. If they won't do, find another supplier.
Get a bag from wilserbag or similar quality.
Start with 7.5 gallons water, use beersmith or online calc to get strike temp. Usually it's near 160 for a 152 mash.
Don't mess with sparging.
After pulling the bag, note volume, you should be somewhere between 6.5 to 7 gallons...squeeze a bit to get in this range. This volume is important to write down, so you can adjust for next brew.
Optional for precision (YOU CAN SKIP) - take refractometer reading before you start the boil. Divide last 2 digits by your target. Yesterday, I was at 1.037 at 7 gallons after lauter, with target 1.049 SG. 37/49 = 0.76 (76%). Then, .76 x 7 = 5.3. So, I'm going to try and get close to 5.3 after boil AND cooling. (Probably want to see above the 5 1/2 line on kettle,since end of boil liquid is expanded, and immersion cooler displaces volume)
OR skip the precision step, and honestly, you'll be pretty close if you did everything ok.
Take a careful, temp adjusted SG measurement, and mark your volume in the kettle prior to FV transfer. My first BIAB I underestimated my boiloff, so I actually added water at this step before sending to fermenter.
Record volume in fermenter.
On batch 2, If you want to skip kits, you can use BIAB calculator or beersmith. I use beersmith BIAB equip profile, and find 69% efficiency seems to be working ok. You'll need those volumes for boiloff and other stuff from the first batch. It's a pain to order grain in weird fractions, so for now, I put the recipe in beersmith with whole numbers, which is how you'll typically find them, then adjust my batch volume to get the SG needed, then follow the instructions for strikewater volume and temp.
Now, I'm thinking about doing a big beer next, and I'll need to "guess" at a lower efficiency, different volumes, etc....but it will all make sense, just start brewing! I suggest an APA in the high SG range to start, these beers are going to be good if it's a bit weaker or stronger than your target.