OK, let's start with seeds! To sow seeds you need special seed compost (John Innes 'seed' is the classic kind). You get a pot, fill it with this (you can mix in a bit of stuff called perlite to improve drainage) and bung in the seeds. For something like courgettes, where the seeds/plants are quite big, I'd start with four seeds per 15cm pot. For tomatoes, where seeds are small, you can sow more in the same sized pot. Water, and then leave them in a place where they will get light and a steady temp of 18-21 C - so an inside windowsill is ideal. Often the seeds need to be covered with a clear plastic bag (I use sandwich bags) to keep moisture in.
Then, when they have sprouted and are a few cm high, you pot them on. For this you use potting compost (I mix mine with perlite again)! It contains more nutrients than seed compost and gives the seeds a boost. You basically gently lift out the seedling with as much of the roots as you can, and repot it into an individual small pot or a module (you can buy trays of these). Water, leave in a warm place again.
Any seedlings that will be living outdoors need to be hardened off. This means putting them in a fairly warm, fairly sheltered place for a week to 10 days - but exposing them to slightly lower temperatures. However, most of the things you want to grow will die if they go below 5 C, so be careful to bring them inside if it's going to be freezing. After the last frost in late April/early May, you can think about planting them out.
However, chillis and peppers will only do well outdoors in the UK in a very hot place. Ideally you need a greenhouse or to keep them inside. This is OK for chillis as they are mostly not gigantic plants, but peppers are bigger. So if you don't have a greenhouse and you don't live in a massively sunny part of the UK, maybe think about ditching the sweet pepper plan, because they are quite big plants and a real risk in our climate outdoors!
Then you're ready to plant out! Tomatoes, courgettes like lots of sun. Tomatoes can be grown in a growbag against a really sunny wall - sounds like you have a good spot on the gravel against your house. Courgettes prefer it in the soil - they will like a LOT of manure dug in and plenty of water.
The exception to all of the above are 'direct sow' things like salad leaves - these are brilliantly easy. You dig/rake the soil to a fine texture, then you just sow the seed directly. Another easy, direct sow thing that is low maintenance is pole beans, e.g. runner beans, borlotti beans, french beans - you should get a really big yield and they look pretty too. Potatoes are also worth doing (but hurry - it's time to get them in now) as they help to break up the soil.
Most herbs need less rich, more well drained soil than most veg - you could use some of the gravel you have dug in and create a separate bed. Bear in mind that some herbs are perennial, and others are annual. It's often easier and cheaper to buy the perennial ones as plants (around £1.50 for a small one in the garden centre) than to try to do them from seed. However, annuals like basil, dill, parsley are easy to grow from seed and really great to have in a large supply.