I'm glad you asked this question before you go any further! I had the same confusion when my children began Latin study. I assumed the stray letter at the end of each vocab. word was the plural, but it is actually the genitive singular. The way you classify a Latin noun into the declension it belongs to is to use the genitive singular. The genitive singular ending of all 2nd declension nouns is 'i' regardless of their gender. So, the student needs to memorize the vocab. word as the nominative + genitive singular. The student also has to memorize the gender of the word because it is not readily apparent by looking at it (though most 1st decl. nouns are feminine, but there are exceptions). If the student knows the genitive singular, s/he will always know which declension the noun is.
I know that this doesn't seem necessary right now, but it is really essential and you will understand why when you get further along. Right now, just make sure your student knows the declension and gender of each vocab. word she learns. She will be so glad later!
I'll be glad to help you whenever you have more questions!