Aphids are a pretty normal problem in early spring and they’ll mostly attack plants under stress.
As basil is a warm weather crop, your plants may struggle a bit until the weather becomes warmer.
The best way to control aphids is with a good neem oil. This is now widely available and has no withholding period so you can spray and eat the same day. It’s best to spray later in the day when the sun won’t burn the leaves. Re-apply a second time 3 days later just in case other eggs have hatched.
You can also make a spray with water and a squirt of a good dishwashing liquid but if too much is applied too often, it can affect the soil.
Planting beneficial flowering plants nearby to build up the ‘good’ bug population can also help with future problems. Alyssum, phacelia and cosmos are good examples.