Is it doable to become an Infantry/Armor officer through ROTC, or is West Point the main way?
Absolutely, ROTC branches nearly the same numbers for Infantry and Armor, the percentage may seem smaller because ROTC commissions nearly 3 times the number of Active Duty LT's then West Point each year. As it was stated above, not everyone wants Infantry or Armor, so do well and your chances are as good as the Academy.
This will all depend on how you place on the national OML. The OML will consist of many things including your GPA through your Junior year, APFT score, Summer training (CLC) scores, PMS evals, ranking in the Battalion, EC's, and other factors. Over your time in ROTC the Cadre and cadets will get you up to speed regarding the OML and branching. There is always an ADSO that can help in getting the branch you want, they will explain this to you later. The sure fire way to get the branch you want is to finish in the top 10% on the Active Duty OML, do this and you are guaranteed your first choice.
Always remember to keep an open mind about the different branches, what you want now may change once you're closer to listing your branches. During Summer Training you will be briefed on the different branches and what they do. It's not uncommon to come back from Summer Training wanting something different.
So for now the best thing you can do is concentrate on doing the best you can in both school and ROTC to put yourself in a position to have choices when it comes to branching.