What are the essential items for a first time overnight backpacking trip?

Even the obvious items aren't. I don't use a tent, for example, unless it's wintertime hiking and it's needed to hold in warmth during the night. I use a tarp or a hammock, occasionally a bivouac sack.

There are endless lists of backpacking gear out there. Google will turn up more than you ever wanted to see. Colin Fletcher's rule was to think of your backpack as your house. Organize it by rooms: bedroom, clothes closet, kitchen, and so forth. Thinking about what you need daily at home is a good guide to what you'll need on the trail. You just have to cut it down to a couple of cubic feet and no more than forty or fifty pounds.

A first time out should always be in mild weather. Pick a balmy summer weekend with no storms threatening. You'll probably survive even if things go wrong, something you may not do in the wintertime.

My gear list for a weekend trip is dozens of items long. I edit it every time I hike, adjusting for locality, anticipated weather, length of stay, length of the trail, whether I'm alone or with someone, and what my interests are at the time (photography, writing, recreation, fishing, etc.). There are certain basics that are always in the pack, but even my first aid and emergency kits will have different things in them depending on the circumstances of that particular walk.

You don't need specialized, expensive gear for a night out in warm, quiet weather. Just pack what you'd need if you stayed home that weekend, go on out there, and you'll learn a lot. Just plan a lot of time in camp rather than on the trail the first few jaunts. You'll be fine.