All good nachos are built on the foundation of corn tortilla chips and cheese. The quality of the end product reflects the quality of the foundation, so it's important to get the right mix. Plain corn tortilla chips are key; please don't make nachos from flavored chips. The mix of cheese is dependent on what's available in your supermarket. Mozzarella and cheddar are good enough, but trade out the mozzarella for monterey jack if you can.
The other toppings are a matter of personal taste and creative freedom, but generally need to cover three bases: meat, green, and heat. That might be ground beef, lettuce, and some chili flakes in the cheese. When I make nachos, I like to add chicken, lettuce, tomato, refried beans, and jalapeño. You can pack a lot of flavor onto the chicken; fry it on a pan with more thyme and cumin than you think it needs.
The most common mistake is forgetting to layer the chips. If you don't layer the chips, you'll have a crust of nachos that comes off as a single mass of cheese, and a load of sad, cheeseless chips underneath. To avoid this disaster, start out with a shallow layer of chips at the bottom of a baking pan, just enough to cover the bottom. Then add some of your cheese and other ingredients (probably not the vegetables, since this is going in the oven). Add another layer of chips, and then cheese. Then chips, then cheese. Keep it going until you've filled the pan or run out of something. Add any remaining ingredients (or any ingredients you don't want to bake) on top.
Bake it at 150° C for 8-12 minutes, just a minute or two longer than it takes the upper layer of cheese to melt.
Nachos are a great side, but there's no shame in having it as the main dish. Serve it with salsa or guacamole!