There are 8 Bits to a Byte.
How mow many data (Megabytes per second) can I transfer on my 100 MBit network?
100 MBit per second, or 12.5 MB per second, theoretically. The real transmission rate is lower. It depends on several factors, such as electromagnetic interferences, transmission protocols used, lossy networks in between, packets being held by routers, et cetera.
What bandwidth (in KBit) does my ISP provide, when my torrent client shows an upstream of 300 Kbps?
kbps means Kilobit per second, therefore it's 300 KBit per second. You later changed your question to ask for 300 KB/s. That's different. 300 multiplied by 8 is 2400 KBit/s.
Your ISP might provide a much higher (upload) bandwidth. You can't just tell that from the upload speed you are currently getting, because it might be influenced by other factors as well (e.g. the bandwidth of the receiving end). Although it's a good approximation, it's most probably not the actual speed your ISP provides.
An ISP will always offer a nominal bandwidth that you can't practically reach.