Answer:
Ok, bear with me this might be a bit complicated.
Explanation:
In order to find he stem of a Latin noun, you need to know the full dictionary entry.
Ex.
amo - full entry amo, amare, amavi, amatus
Then go to the infinitive, or the second part, an take off the re.
Ex.
amare - ama
Then use this handy dandy little chart my Latin teacher gave me to figure out what declension it is.
1 - a 2 - e, eo 3 - e just o 4 -i, io
Use the chart to see what endings match and whichever match thats hich declension it is.
ama - 1st declention
