If you decide to enlist, you can do it on your own individually by making Aliyah. The army will summon you to your first call up, the tzav rishon, usually at least a year after having made Aliyah. If you would like to make a request to have yourtzav rishon earlier, please see - Getting Tzav Rishon.
You can also enlist through a variety of programs. For a list of specific programs, please see the Army Service Programs page. If you sign up for one of the army programs like Garin Tzabar or Mahal, you will do all of your pre army tests and enlist within two to three months of arriving in Israel.
The above was taken from http://www.idfinfo.co.il/ You can visit there for more detailed information.
If you want to “Test the waters” before making any commitments you can go to Keshet Yehuda Pre Military Academy. A 10 month Israeli Army preparation program that will give you a good idea about what you will be getting into. You can learn more here: http://www.jointheisraeliarmy.org/?page_id=18
If you have decided that you are going to join the IDF and would like to prepare spirituality, mentally and physically, learn or improve your Hebrew, get used to the Israeli mentality by integrating into over 130 Israelis with the goal to make it into the best unit they can, then go to this page: