The oven usually gets the blame when cookies come out different every time. The likelier cause is flour: measured by the cup, the same recipe can hold very different amounts, and a kitchen scale fixes that for a few dollars.
Your first day
Pick any basic drop-cookie recipe that lists ingredients in grams (search 'cookies recipe grams'). You'll need a kitchen scale, a bowl, a spoon or mixer, and a baking sheet lined with parchment, with the oven preheated to the recipe's temperature. Weigh your flour to within five grams of the recipe's amount instead of scooping with a cup. Beat the softened butter and sugar together, called creaming, until visibly lighter in colour, about two minutes. Mix in egg and vanilla. Fold in the weighed flour just until no dry streaks remain, then stop.
Bake at the recipe's stated temperature until the edges look set but the centers still look slightly underdone, and don't taste the raw dough. They firm up as they cool on the tray.
Don't keep mixing once the flour goes in just because the dough looks shaggy. Every extra minute of stirring develops more gluten, which is what turns cookies tough instead of tender.
The thing most people miss
It's easy to blame the oven when baking comes out differently each time. But scooping flour by the cup can vary by a quarter or more between two people doing it the same way, which explains inconsistent results far better than oven temperature does.
Weighing dry ingredients means the same recipe comes out the same way twice in a row, and the oven stops being the thing you blame.
What not to buy
Don't buy a stand mixer before a scale. A stand mixer saves effort creaming butter and sugar, but it can't fix inconsistent flour measurements, and that's the actual source of most inconsistent results for a beginner.
You'll know the scale earned its place the third time you bake the same recipe and it comes out identical to the first batch, not just close. Once it does, change one thing on purpose, such as how long you cream the butter and sugar, and note what it does to the texture. A repeatable base is what makes a change visible.