If the preposition 'to' is part of the infinitive, then it's followed by the base form of the verb eg decide to go, want to go, have to go etc. 99% of the time, 'to' is followed by the base form, unlike all other prepositions (for, of, from, on etc) which are followed by the -ing form. The exceptions to this rule are certain phrasal verbs or verb+preposition combinations where the 'to' is actually part of the expression itself, such as your example of the phrasal verb 'look forward to' , or a combination such as 'confess to..'. Other situations are adjective + to combinations, such as 'similar to' (eg Riding an elephant is similar to riding a horse), or the expression 'be/get used to' and its synonym 'be accustomed to' (eg After a while in Britain, I got used to driving on the left).

It's when you can replace (verb)ing with another noun. The pattern "look forward to ___" is always followed by a noun or gerund. I look forward to hearing from you. I look forward to breakfast. I look forward to my holidays. I look forward to the weekend. I look forward to your reply.

For me "I look forward to" is quite formal and the sort of thing I used to write when I was an accountant. Generally I would say "I am looking forward to..." Unless you mean "every year I look forward to my holidays/seeing my family/Christmas etc"