How can I see how many Bitcoins my c-lightning network node has? I am running a c-lightning node and want to know how many btc I am owning on this wallet and how many I can spend. lightning-cli listchannels does not help. What do I need to do?
Use the RPC-Interface listfunds lightning-cli listfunds which lists all payment channels and UTXO. 

If you wish to have a more nicely display you can use this small tool: https://github.com/renepickhardt/lightning-helpers

git clone https://github.com/renepickhardt/lightning-helpers.git

cd lightning-helpers
sudo make install
listfunds


it aggregates everything so that you can see your total funds. Also you can add a parameter to change the unit of display. 

If you don't trust a third party tool and don't want to read the source code you can use this little shell script: 

utxo values: 

lightning-cli listfunds | grep "value" | sed 's/      "value": //g' | sed 's/,//g' | awk '{s+=$1} END {print s}'

total channel balance on your side: 

lightning-cli listfunds | grep "channel_sat" | sed 's/      "channel_sat": //g' | sed 's/,//g' | awk '{s+=$1} END {print s}'