1) but about ebooks, I need to write my own? or would it be okay to forward them free ebooks of others(sure before altering the content, I mean)?
Any free ebooks that train them in affiliate marketing. (ofcourse they shouln't contain links that compete your products.)
2) and please give me a 4 5 lines message example of each of the following you told me that I can get an idea how to write this?
a) promotional material for your product
I have 1 or 2 free ebooks that gives you some idea on how to create attractive headlines and ads. If you want I wil send them to your email.
b)incentive messages on how they are doing
Just encouraging emails and offering a link of their websites in your affiliate members area etc can do I guess.
c) any affiliate ecourses to help them to succeeed in their affiliate marketing
If you want your own affiliate ecourse, you can ask any freelancers to write an ecourse for you. They charge reasonable price. You can send it to your affiliates as series of emails.
3) and how often would be good to send newsletter out, that they don't feel annoying receiving them and do not consider me as a spammer!!!!
It depends... I receive newsleters every day from some internet publishers, some weekly and some very rarely. But one email every 3 - 7 days would be OK, in my personal opinion. Once they opt-in your newsletter, you won't be a spammer even you send emails everyday. All you have to do is follow simple rules like keeping unsubscribe link, providing physical address in emails, provide their ip, email, date of subscriptions in emails you sent.