When should I start to fertilize seedlings?
The best time to start fertilizing seedlings depends on the type of fertilizer you are using. Follow the instructions on your fertilizer package and err on the side of caution, using less rather than more.

If you are using a slow-release fertilizer, you can mix it into the potting mix or soil before planting your seeds. This will provide nutrients to the seedlings as they grow.
If you are using a liquid fertilizer, you should wait until the seedlings have developed their first set of true leaves before applying it. At this stage, the seedlings will have used up most of the nutrients stored in the seed and will benefit from additional nutrients.

In general, it's important not to over-fertilize seedlings, as this can cause them to become leggy and weak.