Ang aking pangalan ay Sam = Ang pangalan ko ay Sam = My name is Sam

Mabuhay! = Long live! - Don't forget the proper sequence of its vowels: A-U-A. “Mabahuy” would most likely get a laugh because it sounds close to the word “mabaho” (stinky). :-)

Ito ang/Itong unang beses.../Ito ay ang unang beses kong pagsulat sa Tagalog sa open forum. = This is my first time to write in Tagalog in an open forum. - You may also use “sumulat” or “magsulat” instead of “pagsulat” for “to write”, however, “pagsulat” gives more emphasis to your act of writing, it being the first time.

Nahihiya at takot/natatakot ako kasi hindi ako masyadong marunong ng Tagalog o Bisaya. = I feel embarrassed and afraid because I don't know Tagalog or Bisaya very well. (“Nahiya” = felt embarrassed; “nahihiya” = feel/feeling embarrassed. “Takot” = afraid; “natatakot” = feel/feeling afraid. I added “masyadong” (very well) because the idea of your sentence won't really be logical without it.)

Ang aking mga kaibigan sa 'Pinas ay nagsasalita ng Tagalog at Bisaya. = My friends in the Phils. speak Tagalog and Bisaya.

Kaunti lang ang alam kong Tagalog at Bisaya. = Ang Tagalog at Bisaya na/Bisayang alam ko ay kaunti lang. = I just know a little Tagalog and Bisaya.

Gusto kong maging perfect/perpecto ang Tagalog ko. = I would like my Tagalog to be/become perfect. (That's why I'm trying to learn it.)

Gusto ko/kong perfect/perpecto ang Tagalog ko. = I would like my Tagalog to be perfect. (That's why I am very careful with how I communicate with it so I don't make any mistakes). ("Kong" would really be the correct form here, but you will often hear us use just "ko" because we are actually saying "ANG gusto ko AY perfect ang Tagalog ko" (What I would like is my Tagalog to be perfect), but we drop the "ang" and the "ay".) 

Ang aking pangalan ay Sam.

Mabuhay! Ito ang aking unang beses na magsulat ng Tagalog sa open forum. Nahihiya at natatakot ako kasi hindi ako marunong magsalita ng Tagalog o Bisaya. Ang aking mga kaibigan sa Pilipinas ay nagsasalita ng Tagalog at Bisaya. Alam kong kaunti lang ang Tagalog at Bisaya. Gusto ko perpekto ang Tagalog ko.

[I'm not sure about my correction. :3]