What kind of punishment may I be looking at for attempting to purchase alcohol as a minor with an altered ID?

First off, admit what you did. You're not going to be the first Soldier to screw up and you won't be the last. What matters is showing that you won't make a pattern of this and that you accept that you made a mistake of your own choosing. You can say PVT or SPC so-and-so convinced you to do it but you're not a robot, own up.
Second, get with your first line leaders. Explain to them you know what you did and that you want to do what it takes to fix this. Hopefully you've already made an impact on them with your work ethic. If not, you've got a fight on your hands to prove that you are worth their time.
Third, contact Trial Defense Services. They're the part of JAG that advises or defends you on adverse actions. Whether you're facing an Article 15 or a Court Martial, don't wave your rights to speak with counsel.

Good luck.

For starters, (as I understand the writing of the Articles) you violated Article 134 in the following sections: False or unauthorized pass offenses (Maximum punishment: (1) Possessing or using with intent to defraud or deceive, or making, altering, counterfeiting, tampering with, or selling. Dishonorable discharge, forfeiture of all pay and allowances, and confinement for 3 years. (2) All other cases. Bad-conduct discharge, forfeiture of all pay and allowances, and confinement for 6 months. ; False pretenses, obtaining services under (maximum punishment: Of a value of $500.00 or less. Bad-conduct discharge, forfeiture of all pay and allowances, and confinement for 6 months.

And that is if your command wishes to push for Court Martial. Otherwise, you could face the Article 15 process which is Summary, Company and Field Grade.

Maximum Punishments In Article 15s:

Summarized: Restriction 14 days, Extra Duties 14 Days, Oral Reprimand or admonition, or any combination
Company: Restriction 14 days, Extra Duty 14 days, Pay forfeiture 7 days; Reduction of 1 grade (E4 and below)
Field Grade: Restriction 60 days, Extra duty 45 days, Pay forfeiture Half month pay for 2 months, Reduction of 1 or more grades (E4 and below)

But, I'm not a lawyer so I have no idea what you would face nor what charges. I searched and found the most applicable Articles in the Manual of Courts Martial based on what you said. You could very well face different charges and face different punishments if it went to Court Martial.

When you swore your OATH you also swore to obey and uphold the UCMJ. You even had to learn the UCMJ for future questioning by your superiors, and while you're assigned 'watch duties'. It makes absolutely NO sense for you to "break any rules", let alone the UCMJ. If you get discharged with anything less than an Honorable Discharge, you will suffer long term damage when you are a civilian again. It's never worth the gamble.

Own up to mistakes. Be honest. Don't give a song and dance explanation. More than likely be on restrictions ( like being grounded) lots of area beautification (yard work) loss of rank (no allowance) til you prove yourself.

CPL (Join to see): Private I send you fervent prayers for future success. I would just learn from your mistake, forget about it, and, move forward into your future.

All depends on your Chain of command.

The better Soldier they think you are the better the result for you.

Be the Best Soldier you can be, take the punishment and move on.

Man up, admit your fault and embrace the suck! Most likely you’ll be maxed out Field grade Article 15. Forfeiture of 1/2 pay, 45/45, loss of rank and a very personal relationship with your CSM; particularly on weekends and holidays!!!!!

I sweat, you aren’t the first not the last Soldier to screw up. Just learn from the experience and get get your act together!!!!!

A significant part of determining what punishment you will get is determined by your reputation and how you present yourself at the article 15 hearing. The best that could be happen is that you are let off with a warning. The worst that could happen is the Commanding officer decide to nail you and refers the case to a court-martial. Neither of those outcomes is likely.

You should have asked yourself the question, BEFORE you altered the ID.

Well a lot of others with opinions here. So I will give you advice from someone who was a lance corporal who did the same thing back in 2003.

I did the same thing I had a fake ID to purchase alcohol as a minor. I never got caught buying the beer with the fake ID I just was an idiot got drunk and lost the ID. They traced it back to me very easily. I fessed right up told them the reason why I had the fake ID then from there was in the Company Commanders officer standing tall. Since I was an outstanding Marine at my job and the war was ramping up My Company Commander recommended Battalion NJP. (Which is what the Army calls Article 15 I still don't understand why they don't call it NJP because it is a Non-Judicial punishment still....)
Went to the Battalion Commanders office reported in and took my lashings like a true marine I received 45/45 and forfeiture of pay since I was becoming a father the Battalion commander ended up not taking my pay and I only received 45/45 and of course busted back down to private.
Learned my lesson and drove on. I defiantly could of been court martialed for the offense but I had great SNCO's looking out for me that had my back.

Plenty of good articles online to help you understand your rights. PM someone (a senior contact) rather than post what could be used as admission of guilt online. Just a thought...