I answered this on the Navy site since I saw it there first.
I am not sure what you mean by being stuck in a basic army unit - 80% of WP male graduates must branch combat arms. About 800 males graduate each year and about 200 of them branch infantry. The rest of the 80 % will branch armor, combat engineers or other combat arms branch.
WP cadets who branch infantry typically go to Ranger school after commissioning, they then go to BOLC (basic officers leaders course) then off to their job as a platoon leader.
No one in the service has to "be stuck" anywhere - promotion and opportunites are by merit. If you do a good job you will get rewarded. This starts at the academy. Right now with the War, there are many opportunities for junior officers.
When you get to West Point you will learn about the branches of the Army, what they do and how they do it. When the time comes you will be able to make a very informed choice about which branch to pick.
