Thanks guys! Maybe I should see someone like that. I know my friend told me to just fill out invoice, but I donīt think I can do it without any bussines number. She also told me that if youīre earning under 30000 a year you donīt have to have a company. That seams even weirder. But Iīm just an artist and this is really another planet for me.....

If your revenue is less than $30,000 a year you don't need to register for GST/HST. Depending where you are you may have to register for provincial retail sales tax.
Assuming you are in an HST province and revenue is less than $30,000 your friend is right - you can just send an invoice. When you file your T1 tax return you need to complete a schedule T2125 where you report your self-employed income and c,laim a deduction for business expenses.