A good place to start is here. You can find a lot of the documents that relate to the conflict and the lead-up to the founding of the State of Israel, which will help you get an idea of how it relates to later conflict. There are also many United Nations resolutions there which relate to the Six Day War, and the subsequent occupation, which cover things like the right of return, the occupations, etc.

The next place I'd look is this site, which includes things like the Geneva Conventions (I think the fourth convention is the one specifically relating to occupations that the ICJ has ruled on), dealing with the border wall (which has to do with the settlements and wall on the occupied territory).

You could also look at the ICJ's opinion, found here.

Some good books to read that will help you get the history are:

• Righteous victims: A History of the Zionist-Arab Conflict, 1881-2001 by Benny Morris

• Palestine and the Arab-Israeli Conflict by Charles D. Smith

• The Palestine-Israel Conflict: A Basic Introduction by Gregory Harms and Todd M. Ferry

Now, if you'd like to read some questions of international law, you're more likely to get good articles out of places like JStor. Some recommendations that will probably help your case:

Israel's Unlawful Occupation by Tom Farer

Issues Arising from the Implementation of Israel's Disengagement from the Gaza Strip by Geoffrey Aronson

Israel's Occupation Of Palestinian Territories Has Damaged Health, Charity Report Says by Owen Dyer (use this one in conjunction with things like the Geneva Conventions)

Hopefully that provides you with a good start. More than that, I cannot give, but you get the idea and a great place to start. I'm not sure how much of this you've researched before, but if you'd like advise on how strong your arguments are for either side, I'd be glad to help if possible!

Edit: Some of this covers the 20-year rule, but I'm hoping there will be leniency because it relates to a conflict that has lasted over the past 20 years and more.