A lot of specialised chilli fertiliser is available and you could spend a fortune on such items. Some people swear by Tomato feed, while others have reasonable success without feed at all.
We would suggest you find a good quality, cheap liquid feed and stick with it for at least the season.
Single part feed. This is feed that is supposedly complete and comes in a single container, like Tomato feed or Chilli Focus.
Two part feed. You will often see liquid or granular feed that comes in two parts. Often referred to as Part A and Part B. These need to be used together, but mixed separately with water. If you were to mix them together before adding to water, some of the content is destroyed or degraded.
To add to the confusion, some feeds have separate growth and bloom feeds. The growth feed is used for strong vegetative growth until you want to force or encourage flowing, when the bloom feed takes over.
What to look for- You will often see feed with three values on the packaging. N, P and K.
N-P-K Stands for:
N - Nitrogen: promotes vegetative growth.
P - Phosphorous: promotes root growth.
K - Potassium: promotes flower and fruit growth.
You are looking for feed with a rough value of 2-1-2.
When to feed
Check your compost or coir etc, and see what fertiliser was in to start with. Often seed starting compost has a week feed that can last 4-6 weeks or more. We would recommend not feeding seedling with anything, as they should have all they need until you pot them on. Once potted on, using fresh compost they will again have access to more feed in the compost. There is specialist feed for seedlings, should you require it.
Once your small plants are in final pots, then you can consider feeding, once the feed in that compost is likely used. Keep an eye on the colour of the leaves, for any deficiencies.
When you begin feeding, always introduce feed at a diluted rate. Too much feed will set your small plants back weeks, so beware. Less, really is more.
Try and keep to a regime. It makes it easier to remember when you last fed and when it's needed next. Follow instructions carefully and never exceed stated doses.
How often you feed, will be down to you and environmental factors like weather and location.
If you are growing hydroponically the below chart may help when trying to gauge the conductivity or strength of your feed solution.