What advice would you offer someone who planned on joining the Army out of high school?

Have a plan....don't get married before 30...if you have a 1st bad duty station at least reup for another one before painting a picture of the entire army.....often times perspective and conditioned thinking can get in the way of things, so take it easy, especially on yourself, you are still growing and finding yourself....listen more than talking.....always be a good battle buddy to all....do not be afraid to volunteer for things, you will look like a kiss ass but you will learn and grow as a leader which is what you want...you want to stand out from the beginning so be visible...DONOT BE AFRAID OF FAILURE!!! See it as learning and improving!
That is how you learn and grow!!! It ain't about how hard you can hit but how hard you can get hit and keep moving forward. take the hits and grow from them...and never take them personal.

Choose an MOS that you enjoy and can do in the civilian world too.

I would make sure the person I was talking to was doing it to further their education, select a MOS that would ensure a future in the civilian life

Take a look at all the branches before making a decision

Pick your job not the hype. When I say this don't take it to be that I mean disrespect to anyone's MOS. We all have our part.

What I mean is you grow up seeing movies, stories, video games about the Soldier. If you truly want to be Infantry then do it, if you don't and you want to learn a different skill like Vehicle Mechanic for example then do that. Pick what's good for you not good for others.

Highly consider an ROTC program, or evaluate your educational benefits depending on your component of Active, Guard, or Reserve. Work on your career path even before you sign the contract.

Enlist for 4 or 6 years depending on your component, don't do a 2 or 3 year it doesn't give you shit in terms of education. Seen a few SM's get boned this way.

Explore an MOS that has civilian application (i.e. linguistic, IT, electrician, civil affairs, etc...). Combat arms (i.e. infantry, armour, artillery, etc..) is sexy to young males, but it can be physically hard on the body and offers few opportunities to pursue off duty education in most cases unlike support duty sections. Of course it boils down to what scratches his itch!

Listen to your NCO's, never quit and keep your damn hands out of your pockets!

Before I went to basic training a friend told me to never ever get in the front of any line. I took his advice and also saw that if I were in the back of a line and did and about face I was in the front. So, I stayed in the middle and watched those in the front be taken out of line for detail of some sort.

Other than that do your best in every thing you are asked to do,

Ask questions like this one. Interview the recruiters. Talk to current military members to get a grad start. Don't ask old farts like me, ask someone who can really give you good advise.

Go Active and stay away from reserve forces.