Assuming you're talking about traveling by airplane:
The first step is to make a list of everything you need and want to bring with you. Once you have your list, start grouping items together by function. For example, put all of your shoes in one pile, all of your clothes in another, etc.
Once you have everything grouped together, it's time to start packing. Start with the larger items first and work your way down to the smaller items. If possible, try to roll your clothes instead of folding them; this will help save space in your suitcase. Be sure to pack any fragile items carefully so they don't get damaged during transit.
If you're still having trouble fitting everything into two suitcases, there are a few other options. You can check if your airline has any restrictions on the number of bags you can bring; sometimes they'll allow you to bring three or even four if necessary. You could also consider sending some of your belongings ahead of time via package delivery service; this is especially helpful if you're traveling for an extended period of time and need more than just the basics.