Maybe what you meant was "pauna", which normally means "an amount paid in advance". In a sense, it may mean "introduction", but we don't usually use it that way. It would be closer in meaning to "for starters" than "introduction". You may see "paunang salita" in Tagalog/Filipino books because it means "foreword". As Kevin has correctly mentioned, "panimula" means introduction or introductory. However, if what you really wanted to say was "introducing myself" then the appropriate word would be "pagpapakilala".
When "taga" is used to mean one's place of origin, a hyphen should follow it: taga-United States
If you found "kinagagalak" in your Tagalog dictionary, then it should be all right. However, I think the word is really "ikinagagalak".
By using "po" in your first sentence, it would mean that you are addressing the person/s with respect. However, when you used "ka" to end your second sentence, you have reduced the level down to the familiar. In order to retain the respect, you should use either "kayo" (2nd person plural) or "sila" (3rd person plural) in place of "ka". The plural forms are used whether you are addressing one person or a group. If you did not have "po" at the start, then the "ka" would just be right.