Tomatoes are hungry plants. As soon as your first truss has set (the moment they turn from flowers into tiny fruits), feed once or twice a week with a high-potash tomato fertiliser or comfrey liquid. Provide sturdy supports: a bamboo cane seems adequate now, but looks puny when dwarfed by the plant in late summer. Tie in the main stem and pinch out the sideshoots.
Mycorrhizal fungi are like magic, extending root systems by many metres, creating a network that sends nutrients and water back to the plant, as they would in a woodland. The RHS recommends them when planting out, but you can also help seedlings get established and nurture them through their life cycle by adding Vaminoc to compost when sowing or pricking out.