Although this seems a bit obvious, the best course when sowing seed is to follow the instructions on the seed packet. At all costs, resist starting seed early: many varieties will grow thin and leggy as they search for light in the still-dark days.
Divert yourself instead by investing in some white, plastic plant labels, or if you are thrifty, make them out of old plastic containers. Copy onto the labels the names of the plants (including the Latin ones) that you will be sowing. Include the initials of the seed company. When the time comes to sow, write down the number of seeds and the date. Although this may seem overly fiddly, this information will be invaluable later on in assessing the performance of your seedlings.
Half-hardy bedding plants such as petunia, lobelia, pelargonium and impatiens can be sown now in a light-filled and warm situation. Use a proprietary seed compost, or ordinary compost with lots of added grit or silver sand. Cover with cling film or a plastic bag to maintain a delicately moist (not soaking) atmosphere.