Go to Ikea and get:
24 wine glasses 24 clear glass salad plates 2 packages of funky paper napkins
After the party you'll put the plates and glasses away and be happy you have them in the future. They're like 99 cents a glass.
2 pounds boiled shrimp in a bowl of shaved ice
mixed pitted olives, some stuffed with cheese
spiced macadamia and/or pistachio (shelled) nuts
3 cheeses on a nice board with cheese knives. 1 mild soft cheese, an aged hard Gouda, and a blue or other moldy type.
None of the above need to be heated or replenished and also don't need to be passed, saving you all kinds of effort.
Pre make silver dollar sized latkes and splurge on an some caviar. Also have a small container of tapenade so people who don't like roe have something to put on it. When guests are about to arrive put the latkes in the oven at 200 degrees. Put on a timer to remind you to check them after the party has started.
Get a melon baller. wrap the melon balls in prosciutto, slide them on toothpicks arrange on a plate and refrigerate. You can do those a couple of hours before people arrive.
That's the minimum to get you through a simple party with a dozen people.
In addition you could do a couple of warm things. The items below are some examples and can be made by a beginner, and you can easily google recipes:
Greek Gyro Meatballs with Tzatziki Sauce
Pigs in blankets, but use chorizo sausage instead of mini hot dogs
Buy frozen filo cups, heat them up. put in a dollop of whipped cheese and a little chutney.
I disagree with people saying you should have a server for a cocktail party with only a dozen people. I think servers with small numbers of guests are awkward unless it's a seated dinner, and they're out of sight for most of the evening. If the crowd is into mixed drinks and you're not confident and it too formal for self service do get a bartender. He/she can also tidy up and do a little general serving with so few guests.