I recommend going to tesnexus and rummaging through the top 100, or, if you're looking for a particular type of mod (weapons, armor, whatever), browse categories. I have no idea what type of stuff you're interested in, so just spend a couple hours looking through it for a few things that sound interesting to you.
Most mods you just have to unzip into the data folder. That may be the easiest way to go since you're just starting, but I recommend getting wryebash and using its BAIN feature to install your mods (it makes it much easier to uninstall them). Actually, I recommend you familiarize yourself with wryebash, boss (better oblivion sorting software) and obse (well, just install obse). TESCOSI is a good site to get smart on this stuff, I think. I'm not providing links, but you can easily find this stuff with google. Hopefully that helps you a bit.