"I am all for looking after the environment" is correct. It's a little informal to say "all for <doing something>", so "I'm all for looking after the environment" is better. "I am supporting to look after the environment" is not natural. You don't use the infinitive after "support", you use the present participle, so "I support looking after the environment". "I am supporting the environment" is also not natural. The present progressive ("I am <doing something>") doesn't work here, because you are talking about something you do in general, not an activity you are doing at the current time. "I support the environment" is natural.