Firstly make a few lists of what you need to take. You may want to make a clothes list and then another items list. Once you have this you can then start to work out what will need to be packed into which case. Some items, for example, may have to go in your carry on luggage whereas other items will need to go in your hold luggage. This is very important as once your suitcase has gone if you then discover something in your carryon luggage that needs to be in your hold luggage you will often have to dispose of it.
Make a separate list of items that you may not be able to pack in advance such as phone chargers, toothbrushes and hair brushes etc. You can then refer back to this list the day you are due to travel to make sure you have everything. Making sure that you have all your travel documents in place is very important. You will need to check that you have a valid passport, copies of insurances, visas (if required) and details of your accommodation and flights. If you are taking prescription medicine then you will need to ensure you have a copy of your prescription with you.