Oh, finally a question I know something about. So, Bohemia as a kingdom existed from year 1212 roughly in the area where Czech republic is today. But in year 1212 bohemian rulers were only given hereditary king title. However the Přemyslid dynasty (in czech Přemyslovci) ruled Bohemia from approximately 9th century.

The Přemyslid dynasty is considered the founding dynasty of the czech state. Before year 1212, bohemian kings acquired kingship twice, but the title was lost after their death. However, in the beginning of the 13th century, Holy Roman Emperor was fighting against Pope in a bloody struggle. Bohemian duke Ottokar I. of Bohemia (in czech Přemysl Otakar I.) switched sides few times before definitely supporting Emperor Frederick II. in the struggle against Pope. For this, duke Ottokar I. was rewarded with hereditary kingship for Přemyslid dynasty, therefore becoming king Ottokar I. So that is how bohemian dukes became kings.

When you speak about downfall of the Přemyslid dynasty, there are two things I should mention. In the second half of the 13th century, czech king Ottokar II. tried to become an Emperor of the Holy Roman Empire. These Emperors were elected by several important figures from the the HRE. One of these voters was for example czech king, others were important dukes and bishops from the HRE. At that time, Ottokar II. was one of the most powerful rulers in Europe (if not the most powerful). He was able to expand the kingdom quite a lot. He almost gained an access to Mediterranean sea (not bad for middle European country, is it?). So when he saw a chance to became the next Holy Roman Emperor, he took it. He was sure that he will be elected as the next emperor, but the voters elected Rudolf of Hapsburg, who was in that time absolutely futile in power and the name Hapsburg was unknown. The voters were probably scared of a strong Emperor, which is probably what Ottokar II. would become. Ottokar II. was absolutely furious. He wanted to fight for the throne that was still his in his mind. All this led to the Battle on the Marchfield in year 1278, where Ottokar II. was defeated by Rudolf and his allies. In that battle, Ottokar II. was killed and bohemian lands were without Přemyslid rule for several years.

After the death of Ottokar II., bohemian lands were under the foreign rule for few years and later Wenceslaus II. came to power. Under his rule, the Bohemian kingdom became probably the wealthiest kingdom in Europe for that time, thanks to silver mining mainly in the city of Kutná Hora. He was able to gain the polish crown as well as the hungarian crown for his son, Wenceslaus III. However, he died quite soon after his successes. This sparked a revolt among the Polish and the Hungarians, which Wenceslaus III. tried to defeat. However, on his way to Poland, he was murdered, thus eliminating all the men of Premyslid dynasty. To this day, we do not know for sure who the murder was or whom did he serve.

I have written this in school (in quite a haste :)), I hope that it is at least somehow helpful. Feel free to ask any questions, I will try to answer them all. Please excuse any grammar mistakes, english isn't my native language.