Normally, you can continue merging 2 items of the same kind until they form the item you need to complete the task on the right part.

You can also click on the icon of the item you need to create to know the exact number of merges you need.

If your board got full, you can delete the tiles you don't need by dragging them into the Delete/Trash can icon. Make sure to hold press the tile so you can properly drag and delete them.