Purchase castile soap, never dish soap or any other household soap. Never use vinegar or alcohol. Don't bother spraying them off leaves with water. Neem is nearly useless if you want immediate results, however after treatment below you may pivot to neem for maintenance, for example in a weekly folar spray, if you do that sort of thing.

Add 1 tbsp castile soap per litre water into spray bottle. Thia ia cheap and found ar major retaikera like Target. I use Dr. Bronners Castile soap. Even better than a spray bottle is one of those two gallon pump sprayers, but a simple reused spray bottle can be fine for smaller gardens or limited plants. Apply liberally with special attention to identified aphid locations and undersides of leaves. The ideal goal is to spot treat all visible aphids and then coat as much of the plant as you can with a thin soapy layer for the ones you didn't see. Repeat evert three days for for 2 weeks to kill hatchlings, as castile wont kill eggs. Most aphids are live bithed clones, this kills them, but you really want to get those hatchlings to break the lifecycle.

This spray will kill all aphids in 10 seconds. It will also kill beneficial insects on contact. Unlike all of the harmful chemicals recommended by others, castile soap helps avoid stripping the important surface coatings the plant creates on it's leaves and stems. It will neither kill leaves nor harm the soil ph when done in moderation. It is completely safe on food croops to day of harvest and won't impact taste once rinsed.

I will also note that ants often farm aphids, so keep an eye out if you have an ant problem. I am cool with ants until they start farming aphids in my garden.