Firstly,
Welcome to the forum!
Secondly, you are in the right place when it comes to mission editing in OFP.
Thirdly, it's not difficult but it does require a certain amount of application at the start.
Now, head over to the Editors Depot (link at the top of this page) and get stuck in. There is a lot to absorb and I recommend that you start with Tutorials - Getting started. The best things to read first are snYpir's OFPEC Guide: Making missions and my own macguba's guide to mission editing for beginners, which I wrote especially for people in your position.
As a break from reading go back to the Mission Editor and play. Click on the Units tab, then double click on the map.
Once you have a unit in place, click on it; click on the waypoints tab; and click somewhere on the map. You're off. It gets really fun, really fast.
For the avoidance of doubt, creating your own mission on an existing island is one thing. Creating your own island is quite another. Start by making very simple missions for your own amusement and build up from there.
Edit: beaten by j-man