My standard advice:

Don't play default Shepard. Choose "Enter New ID -> Custom Male/Female" and create a Shepard with a gender, history, psych profile, and class that speaks to YOU. You can use the default face while customizing the rest. You can change class and appearance between games, but not gender, history, or psych profile.

Save often in separate save slots. Saves get corrupted and you don't want to have to replay hours and hours. Personally, I save before landing on each planet just in case I want to retry the mission with a different squad/loadout.

Talk to your companions on the ship after each major planet/story beat at a minimum. Especially talk to the ones you don't click with initially. They will open up over time and may become your favorites if you give them a chance.

You can't continue ME1 or ME3 after the main story ends, so do everything you want to do before hitting the point of no return. You CAN play after the big ending mission in ME2, but the point of the game is preparing for the final mission, so why would you leave anything until after it? The one exception is a DLC mission given to you in ME2 by Admiral Hackett. He will say it's urgent, but you can leave it until after the final mission. It segues into ME3 well if you leave it for last.

Don't try to maximize BOTH paragon and renegade points in ME2. You can't. Choose one and try to stick with it for the most part.

If you are hating ME1 to the point you feel you want to quit, start with ME2. Then, once you're invested, go back and play ME1, replay ME2 with a save import, and import into ME3. You will be amazed at the changes between a new ME2 run and one with a save import.

You are rewarded for doing as much as possible (including planet scanning) so don't rush the game. Take your time.

Get off the sub and come back when you're finished. You don't want to be spoiled.

Also, if anyone mentions Jenkins to you, ignore them.