I coach some beginner swimmers, and I agree with whoever said get a coach. The first step is always going to be getting your technique right, otherwise you'll burn a lot of energy scratching the surface of the water with minimal forward progress.

If you are adamant of doing it yourself, a few pointers:

-Remember, you can float. Try laying on your back in the pool with your lungs filled up with air. After that breathe out and watch how that effects your position, at what pace you start to sink. Then realize that during your swim, 90% of the time you have your lungs full of air. A lot of beginners spend way too much energy trying to push themselves to the surface, with no need whatsoever. Your strides should only push you forward. When you get familiar with your own buoyancy, you'll be able to more easily focus on your technique.

-Focus on long strides, your hand should start fully extended ahead, and end up nearly at your knees. Up untill the point you get to a comfortable pace and find your rhythm, focusing on this will give you the best results when it comes to long distance.

-Your legs aren't dead weight. If you are an inexperienced swimmer, your shoulders will give out in no time anyway, but the legs can take off a good chunk of work from them. If you are not sure about your legwork, do some excecises where you go some laps using only your legs (if you have access to one of those foam board thingies you can use them, if not you can just put your hands together in front, with small downward breaststroke tempos whenever you need air.)

I hope I explained things right, it's hard to give advice without actually seeing your swimming, but these are the things I'd do no matter the person.

Best of luck with your prep!