Computer issues help me!!!!!

Hello,
my desktop turned black and all i have is the bar with start im only 15 my dad gave me this computer brand new i rarely use it and im supposed to fix it myself i dont know what to do it says i have threats infections and worms and i dont have money or credit cards what can i do i have downloaded everything now i have the big problem cuz whatever i download i cant see what do i do to fix this?????????? everything that scans tells me at the end to give it my credit card numbers and they will bill me and fix my computer on a daily baises but i do not have money