Now is a good time. That way the food is available to them when they wake up which they should be doing about now anyway.

Potted inside a greenhouse or outside? Or in ground plantings? In general, use a slow release fertilizer appropriate for what you are growing

Thanks will start this week

Depends on the plants and what feed you're going to use - as Loosestrife says, a general purpose slow release one such as Growmore is the most suitable for this time of year, and for any plant.