There are a few possible causes for leaves curling up, especially on tomato plants:

Watering: Sometimes leaves curl up during periods of hot, dry weather - it's a way to limit how much water the plant loses. Plants can dry out very quickly in hot weather, especially if they're grown in pots. A larger pot may help, as small pots dry out faster.

Alternatively, too much water can cause similar symptoms. The key to watering is to poke your finger into the soil or potting mix. If it's dry about an inch (2-3cm) below the surface, then it's time to water.

Pests: Bugs like aphids can cause curling leaves. If you look at the underside of the leaves, you may find the culprit.

Disease: Sometimes diseases can cause leaves to curl up. The plants will usually exhibit other symptoms such as discolored leaves.

Fluctuating temperatures: Curling leaves can sometimes occur if temperatures fluctuate between warmer and colder (for instance hot days and cool nights). This isn't serious but covering the plant up if possible (at least overnight) may help.