As long as you have a proven track record of academic success you should be competitive for a two year. I have seen pretty much all college programmers pick up at least a two year scholarship; however I have seen more pick up a three year scholarship, although all of those students were a college programmer since their fourth class year. I would recommend talking with the NROTC unit before the start of your junior year. If you just finished up your sophmore year now, speak with them in the next few weeks, as it will help them get to know you and work effectively to get you a scholarship. You would need to have your citizenship status solidified before then, and it can be a somewhat lengthy process, so start it sooner than later.