Ordinal numbers can be used to determine a sequence. The ordinal number represents the position of an element in the sequence. For example, the days of the month are a series of numbers in which one day ranks in a certain position:

Example:
der siebzehnte März = the 17th day in the month of March
der dreißigste April = the 30th day in the month of April

In German, ordinal numbers are written as digits followed by a period:

Examples:
der 17. März = March 17th
der 30. April = April 30th

When spoken or written out as a word, they are formed as follows:

For the numbers 1 to 19, the ending -te is added to the cardinal number in the singular.
Exceptions for ordinal numbers are "eins", "drei", "sieben" and "acht".

Combined ordinal numbers greater than 13 are formed the same way as cardinal numbers:
First you say the hundreds, then the ones, then the tens. Only the last part is given the ending for ordinal numbers.

More:

Ordinal numbers usually precede the noun they describe. They can be used with definite or indefinite articles - or no article at all.
Ordinal numbers have different endings according to gender, case and number and the accompanying article.

Example:
Das erste Problem: Heute ist der 31. April.

But:
Ich sehe Sie heute zum ersten Mal.