No worries Zand I was as clueless as you were when I first tried this stuff out (just read my post history to see some dumb shit I did and asked). We're here to help.
1) Find your UFO Defense disc or contents. As a source-port you need original files from the original disc. UFO Defense can be bought legally from a variety of sources, including Steam and probably others like Good Old Games.com (gog.com) and gamersgate (gamersgate.com). Unfortunately I don't think any of us will help you if you ask to pirate it or find an abandonware copy somewhere.
2) Download and install the OpenXcom (often abbreviated as OXC) milestone 1.0 1.0 will ask you to point out the folders with the original UFO defense folders. IGNORE THIS STEP (I'll explain why in just a second).
3) Download the latest nightly. OXC 1.0 is showing a bit of age now compared to the Nightly builds that have come out. The Nightlies are as simple to install as downloading them and extracting the contents of the nightly to your OXC folder, overwriting anything it asks about. They are also required for many mods now.
4) The latest nightlies necessitated a somewhat big re-haul to the 1.0 OXC system. Originally, we just dumped shit into a Data folder (which was simplified by the installer pulling the folders it needed and automatically dumping it into the data folder). With the TFTD (Terror From The Deep; the first sequel) support version coming out soon, the Data folder is now obsolete. This is because TFTD folders and UFO Defense folders must now be separated. The old Data folder must be deleted.
5) Now that you have installed OXC 1.0, extracted the latest nightly, and deleted the obsolete data folder, it's time to find your raw UFO Defense files. Open up your OXC folder. Locate your UFO folder inside OXC. Copy and paste the following folders from your raw UFO Defense folder: GEODATA, GEOGRAPH, MAPS, MISSDAT, ROUTES, SOUND, TERRAIN, UFOGRAPH, UFOINTRO, and finally UNITS.
6) At this point, your basic OXC should be ready to rock'n'roll. If you want, give it a test run and ensure it loads properly. That being done, it's time to help you with mods. Luckily you are a fellow Windows user (even if you are running Shitsta) so I can help you with your mods. Go into your computer and locate your username folder or your myuserdocuments or whatever Windows Shitsta has it named. You'll know you found what you're looking for when you find a folder named OpenXcom. This folder contains your saved games. Inside it, create a new folder called mods.
7) Open up this mods folder and place the mods you want. Make sure the mods you place are un-zipped. For example, I have the shotgun and sniper rifle mods placed there. When I launch OpenXcom and look at my options menu and go to mods, I'll see the 2 mods I downloaded listed there as toggables. Once you set a mod to on or off OXC will restart to take effect. This is completely normal.
Have fun and let us know if this worked!