People from some countries are allowed 30 days without a Visa. Others are allowed 6 month tourist visit with out special Visa. Contact you local US Consulate or visit the US Imigration website for info on your country.
If you are just going to visit him in the US, all you need is a visit visa (tourist visa) but if you are planning on making your home permanently there with him, he needs to apply for a K-3 spousal visa for you.
To apply for a visa to visit the United States, a resident of Uganda would need to visit the US Embassy in Kampala. There it is possible to fill out the necessary documentation to receive a tourist visa.