That has yet to be decided - the vote will be held some time in 2018.
There is a feeling in the IOC that the fact that the Olympics have never been held in Africa is a serious omission, so if any African city can mount a credible bid, they will be a shoo-in. The most likely such bid would come from South Africa, based on either Cape Town or Durban. South Africa did a lot of preliminary work prior to the vote for 2020, but in the event pulled out. Whether they feel they're ready now remains to be seen.
If not Africa - there is a reluctance to hold two successive games in the same continent, so Tokyo 2020 would exclude any Asian city from being awarded 2024, which means that we are left with Europe or North America.
The two cities which bid unsuccessfully for 2020 are Madrid and Istanbul. The recent political turmoil in Turkey might put the kibosh on Istanbul having another go - whether Madrid will try again, I can't say.
Edit: - further searches, courtesy of Google, reveal that there is a shortlist of four serious candidates for 2024. Sadly, none are African. The short-list is Budapest, Paris, Rome and Los Angeles, with only Budapest never having been awarded the games before.
Cities which (with varying degrees of realism) considered making bids, or who have withdrawn bids, include Boston, Washington DC, Berlin, Hamburg, Baku, Istanbul, Kiev, St Petersburg, Durban, Johannesburg, Doha, Nairobi, Casablanca, Madrid, Taipei, Singapore, Kuala Lumpur, Nairobi, Busan, Guadalajara, Delhi, Brisbane, Melbourne, Lima, Bahrain & Toronto.