Tomii,
Hiragana is used for Japanese words in Japanese. Katakana is used for "borrowed words", that is, words brought in from English (and elsewhere) such as escalator, elevator, etc. As you have noted, most foreign students learning Japanese write their names in katakana, as their names come from a foreign language.
" ... can you have conversation just using the 2 or doo you also have to use kanji?"
--> As Godppgo stated, in conversations, we speak, we do not write, so characters are not used in speech. For example, if you want to use the word for "study" in a conversation, just say "benkyou suru", do not worry if it is written in hiragana or kanji.
" ... and how do you use the sybols for japanese?"
--> Can you be more specific?