If you already have a suitable tow vehicle, you must be aware of the tow capacity & payload of your vehicle to make sure you know the limits & ensure it is not overloaded.
Set yourself a budget that you are able to spend & be aware of the extra costs involved. There may be accessories required for your tow vehicle such as a heavy duty tow bar, sway bars & hitch, electric brake controller, reverse camera, etc. There are also extra items for the caravan like power cords, water hoses, shade walls, etc.
Decide if you are happy with a used caravan or prefer to purchase a new model. Obviously there are price advantages to buying a pre owned caravan & they will often come with all the little extras you need, however, sometimes finding the exact one that suits you can be difficult. As caravans hold their price fairly well, the used vans are often not much cheaper than buying a brand new caravan.
Think about what sort of places you would like to visit & stay. If you are a beach person & love the coast, there are numerous caravan parks & camping spots that you can stay. There popular places are usually all reached easily on bitumen roads & have plenty of facilities to use. If you are more of a bush person & want to get away from all the people & explore the outback, you will need to buy a caravan that has off road capability. You will also need to plan for more water storage & increased power supplies with large battery & solar systems.
Make sure you can sleep everybody that wants to come with you. Are you just travelling with a partner or do you want to take children or friends with you? There are layouts available that cater for extra people.
Plan where you are going to keep your caravan when it is not being used. There are numerous storage facilities available if you do not have room to leave your caravan at home.