Welcome to the forum!
You are on a well worn path ... try a bit of OFP editing, find OFPEC, get completely overwhelmed by the amount of information here. ;D
Head over to the Editors Depot (link at the top of the page) and look down the menu on the left hand side till you find "Browse Tutorials". Click on it and use the drop down boxes until you find the category Getting Started and the sub-category First Steps. There you will find tutorials by snYpir and my humble self that will set you off in the right direction. You don't need to read all the tutes in Getting Started, but I strongly recommed you have a look at several of them.
Now, to answer your question specifically. First, make sure that at the top right of the Mission Editor screen (the one with with the map) it says "Advanced". If it says "Easy" click on it to toggle to Advanced. Everybody uses Advanced - Easy just switches off some things which is no fun at all.
Now create your unit .... when you do so a screen appears. About 1/3 of the way down there is a line called Initialization. In this box write
removeAllWeapons this; this addMagazine "XXX"; this addWeapon "YYY"
You need to have as many addmagazine commands as you want magazines. Make sure you add at least one magazine before you add the weapon, otherwise the weapon will not be loaded at the start of the mission.
XXX and YYY are obviously the names of the magazine and weapon you want. There is a list in the Editors Depot - Tutorials - References - Weapon and Ammo lists. The best one is
here.Ammo crates in general don't have guns, only ammo. However, you can add weapons to a crate in exactly the same way as you did for your loon. Crates have lots of space so you don't need to remove whats already there.
Two more tips. If you haven't already, update your version of the game to 1.91 from the official site at
www.flashpoint1985.com. And if you are an experienced coder you will find the
online command reference particularly useful.