What number do you need to dial an American cell phone from Ghana?

To dial any number in the USA, Canada, etc., from a landline in Ghana, dial 00 1 and then the three-digit area code and 7-digit subscriber number.

To call US/Canada/etc. from any GSM mobile anywhere in the world, dial +1 (including the plus symbol) and then the area code and number.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but Nigeria uses 009, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)