What's more common way to ask "can I"?

HI Wladuszka,

You could say 

‘Je peux vous aider?’ = Can I help you?

With your voice rising at the end to indicate that it is a question and not a statement... 

Take a look at the following lesson for more examples -

https://progress.lawlessfrench.com/revision/grammar/three-simple-ways-to-ask-yes-no-questionsintonation-est-ce-que-nest-pas

Hope this helps!