I would just start by reading the basics so movement, shooting assualt and just read it again.
Look online at some battle reports that helps as well. I find the special rules are where there is a lot of confusion when learning at first but don't get to overwhelmed.
I would just play a few small games 500 points maybe and just learn how it plays and if any questions come up google is your friend. If you can't find an answer Dakka will help you out for sure.
Bottom line is just read the codex and the rulebook a few times (not in one sitting
btw) and then just play some games together slowly building up to larger games and understanding the special rules better.
Go on youtube and look up miniwargaming how to play
40k it gives a good breakdown of how to play the 3 main phases for starters hope that helps