The minimum qualification for the non-commission officers is SSCE while the maximum is diploma certificate. The maximum rank for this is Master warrant officer. If you are able to further and get a BSc or HND, you will then apply for presidential commission to the rank of lieutenant as the case may be.

NOTE
Nobody should deceive you that without godfather you cannot further your education. Its a BIG lie.. I know a lot of other ranks who are BSc and MSc holders. Note that your salary as SSCE holder in the military is a level 8 basic salary of x amount or there about.

We have tradesmen and non tradesmen. If you join as a tradesman, that's, if you have the skills needed in the military like technicians, plumbers etc, you might be given the rank of LCPL or CPL on passing out. If you join as non tradesman, you will pass out as a Private.

Furthering your school while in service depends on many factors. Your unit can release you to study and you can as well do it on your own. If your unit is to release you, you must have worked for some number of years and attain a certain rank. If you are to study on your own, it depends on your unit Commander, CO etc. If you are attached to work with a very senior officer, he can use his position to release you to study.
Promotion in the service has nothing to do with your civil certificate. When you join the military, you will start attending military courses and promotion exams. You must be disciplined to the core to avoid having a regimental entries in your file to affect your promotions.
COs and Commander's good recommendations are what qualifies you to be promoted. The highest rank attainable for soldiers is AWO. Army Warrant Officer. Master Warrant Officer is majorly the rank soldiers retires on.
AWO rank is very few in the Army. Its for those who exceptionally distinguishes themselves in the service. Promotion is every four years if you qualified. At same time, you cannot rule out man know man. If you want to enjoy promotion to your next rank as when due, join the Air Force. For the Army, it will only take the grace of God for you to reach your desired rank because, you strives to be distinguished in the Army.

Pick the form
Register online
Wait for screening (Before it use to be state screening before zonal screening, but now I think it's only zonal screening that you will undergo)
Screening? Running, Medical Examination, Screening of Certificates, that may not be the order though.

Tribe does not affect, since each state has its own quota.

When you join with SSCE, you're a recruit. After POP, you'll become a Private.

They will check your BP,
Check your anus (for pile) check your balls/breasts ( to make sure it's two)
Check if you have been operated before. Attitude test, x-ray test, oral test and physical test (running).
What drops most people is fear of not been picked, which will make there BP to rise, and once you have high BP you will be disqualified. If you qualify for the training, the training is in Zaria (depot Nigeria army) you will be welcomed with frog jump from the first gate to the parade ground.

After three months you will be paid 3months allowance (arrears) and then you will be recieving monthly allowance every month for three more months. The first two weeks will be hard and painful for you, but if you persevere and endure the pains without running away from training for a month, the rest will be easy, the more the months, the less the stress. You will be a private soldier (rank) with a salary of xk.

Your tribe doesn't stop you from joining, just that it is more competitive, out of 1000 potential recruits, only 90 or less will be taken. As for getting commissioned later, you need to have Bsc or its equivalents, serve the army for 15 years (I heard it have been reduced to 12) with the rank of a Sgt before you can apply for Executive commission, the training will take 3months after which you will become a full lieutenant. Your rank won't exceed major before retirement. But if you don't go for the commissioning, your highest rank will be Army warrant officer (AWO) which is hard to get, but you can become a master warrant officer, that will be after many years in the army.

In those days it was possible, if you're determined and are lucky to have a good boss. But you'll pass through a lot of challenges. Now, with the insurgency, Fulani herdsmen, and other internal security challenges, it has become even more difficult.

No promotion for additional qualifications acquired on the job, but after earning your degree or HND, you can go for DSSC. No special consideration will be given to you though.

With SSCE, its Army Warrant Officer (AWO). Only a few people attain such. The probability of you reaching that is very low. 4 or maybe 5 per intake.

if you join with your ssce, you will start as a private, that is the lowest servile rank in the army. It's also possible to pursue a degree while in the army if you are fortunate to have a commissioned officer that can lobby you through. After you have obtained your degree you can convert to commisioned officer rank through DSSC or SSC; you start with the rank of a second lieutenant

The question is, are you ready to die for Nigeria? If No, please consider an alternative career. Do not join the military as a last resort or as a means to escape unemployment. if your crave to join the Nigerian army is driven by a burning passion for adventure and military regimental life, then I say to you go for it; but if you are driven just by the need to get a job and escape frustration, then I suggest you have a rethink because you won't like the job at all and you will consequently under perform.

Follow your mind.