Something is first if it comes before all other things. When you win a race you come first. The picture shows the first of four candles alight.
A second is a unit of time. There are 60 seconds in a minute. Second is also the number two place in a competition.
Third comes after second, being the number three time something happens. You might come third place in a race, be third in line, or be like the little girl in the picture, having a third birthday party. See also thirds.
Something is fourth if it is the number four thing in a list. Fourth comes after third and before fifth.
Something is sixth if it is the number sixth item in a list. Sixth comes after fifth and before seventh.
Something is seventh if it is the number seven item in a list. Seventh comes after sixth and before eighth.
Something is eighth when it is the number eight thing in a list. Eighth comes after seventh and before ninth.
Something is ninth if it is the number nine item in a list. Ninth comes after eighth and before tenth.