The standard for making international calls is to dial 00 + country code + area/city code + telephone number.
From a mobile phone, you can dial "+", country code (1 for US and Canada), then the 10 digit number.
Calls to North America and to most countries overseas by mobile phone can be the same rate as a local call (as low as 3 cents/minute), depending on your service provider. The charge for calling other countries varies, depending on the country, but is usually fairly inexpensive. Check your service provider for exact rates. It is generally cheaper to make calls from Ghana, than to Ghana (from abroad).
Alternatively, a call via the Internet can be free (except for the data used to make the call.) The best-known provider for this is Skype, but there is also Google video chat, and others. To save on data usage, and for a better quality of call, consider turning off the video option, and just use voice.
Via Magic Jack: A device that uses the Internet to make unlimited calls to the US and Canada. You can set up multiple devices (computers, phones, handsets) to one Magic Jack and make multiple phone calls at the same time. Requires a yearly subscription. A few people we know use it and report having no problems with it.
From overseas calling Ghana, friends and family can reach you by dialing 00 (North America) or simply entering the + sign, 233, then your 10 digit number without the 0 at the beginning. They have several options for how to reach you; international calling card, Skype computer to phone (requires credit), or other VoIP options. As far as a good phone service for family and friends to use while trying to reach you in Ghana, Vonage has been recommended as having good rates. They should use extreme caution if calling through a regular North American mobile phone service, as rates can be as high as $2/minute or more.