I have been planting seeds for a few weeks now, and things are starting to happen. But don't worry if you've not started yet, there is still time to start most seeds off, and some still are best left till later in April or May. Check the instructions on the packets and start sowing.
I've only started hardening off some hardy wildflowers as it is still quite chilly, and my seedlings are still very small; so I'm just potting up to give the roots some extra room. I use the veranda at the back of my house, as it has a clear roof, but I do just as well on my sunny kitchen window ledge, with the long thin propagator boxes.