Leggy seedlings are a common problem when germinating from seed, especially early on in the year. The problem is caused by uneven light. Perfect plants purchased from garden centres are germinated and grown in ideal conditions, just the right amount of heat and humidity, and critically, all round light, that is 360°. As the plant grows it gets all round light and develops a straight study stem.
Most gardeners have less than perfect conditions and germinate seeds on window sills and warm spots in the house where the light is predominately from one side which causes the seedling to pull to the light. Combined with poor light levels, as is often the case early in the year, or during poor weather, and the seedling stretches to the light and looks "leggy", or weedy. The image below left are my tomato seedlings with just that problem.
You can reduce the changes of the seedling becoming leggy by ensuring you germinate it in the lightest possible conditions. Also turn the container every couple of days so the light is not constantly drawing on one side of the seedling.
But what if you have done this and still got a weedy seedling. The solution lies in how you pot on the seedling. To pot on, ease the plug out of the pot from the base, and pull the seedlings apart gently (as in image two.) Avoid touching the stems which are very delicate and if damaged the plant may not recover; handle carefully and hold by the leaves.
Select a suitable pot, not to large. It's tempting, especially with plants like tomatoes which will need to be potted on several times before eventually making it into the veg plot or grow bags, to put into a larger pot to save time. When potting on it is important to pot up only a slightly larger container. If you put a small seedling (or any small plant ) into a large pot it will not thrive.
When you put the seedling into the pot sink it well below soil level, you are in effect burying part of the leggy stem under the soil. Instead of potting on at soil level, reduce the amount of leggy seedling above soil, and you will find it grows on as a much better seedling it's leggy past forgotten. Images centre and right show the same tomato seedling after it has been potted on with several cms of (leggy) stem below soil level. As you can see from the image the seedling looks good and this tip works well to compensate for the problem with all types of leggy seedlings.