well in economics you learn about the economy, thing such as inflation, unemployment, economic growth and things like that. it is an interesting course because once you understand it, you understand most of what is going on with the economy, for example, you will learn the economic theory behind why it is good for the government to spend money like crazy when the economy is doing badly, such as right now. you learn other stuff to about the value of the dollar and how that affects the economy as well as prices of products and just a bunch of stuff like that. even if you dont intend to follow economics through into university (which i didnt, im doing science) it is still an good subject as it helps you better understand whats going on in the world.
im assuming that modern history is like ancient history. you learn about a person, then a historic event, then a historic period of time and stuff like that. 4 topics each year. the choice about what you do is extremely varied if you check out the syllabus, but your school will probably only teach you certain subjects so it is best to ask your history co ordinator to see if you will like the topic areas being taught. BoS wont be a great deal of help on this one
and I did chem. its pretty good. somewhat interesting if you like science, which you seem to. you learn about chemical reactions and equations and such. in prelim you learn about metals, energy, water and one other topic. this is one where i would say a teacher can really influence how interesting the subject is. a good teacher can make it good, and bad teacher can make it terrible