“Ang gagawin mo ngayon” = What you will do now/today.  -  This is correct, but I think you meant “I” and not “you”.  “Ang gagawin ko ngayon” = What I will do now/today.

Your use of “ngayon” above to mean either “now” or “today” will depend on the time of day.  If you are saying this in the morning, it can mean either way.  If you are saying this in the evening, it will be more of “now”, to mean the remaining hours of the day.    

“Ngayon, tutugtog ako ng plauta at magko-compose ako ng kanta para sa plauta at gitara, pagkatapos pupunta ako sa” – up to this point, everything is correct.

“ang kaarawan na parti” = the birthday that is a party.

Any of these will do as ending for your sentence:

“parti/party para sa kaarawan ng kaibigan ko.” = party for the birthday of my friend = my friend’s birthday party.

“parti/party ng kaibigan kong may kaarawan ngayon.” = party of my friend whose birthday is today.

“isang birthday party.” = a birthday party.

“birthday party ng kaibigan ko.” = my friend’s birthday party.

Ang gagawin mo ngayon

Ngayon tutugtog ako ng plauta at magko-compos<em>e/ gagawa</em> ako ng kanta para sa plauta at gitara <em>at </em>pagkatapos<em> ay</em> pupunta ako sa ang kaarawan<em> na may salo-salo ( or you can just say "salo-salo")</em>.

If you wanna elaborate say " <em>Pupunta ako sa salo-salo dahil kaarawan ni Juan</em> "