I am trying to say that saving the environment will guarantee a future for future generations. Does it actually mean that or have i made mistakes?
Yes, it's right. But, it's much better to say, 我认为保护环境就是保护子孙后代的未来.
保证未来 doesn't sound a natural phrasing. Usually, we say 对未来的保证 or 是未来的保证. A more natural one can be 我认为保护环境是对子孙后代未来的保证.
The other way to express this kind of idea in Chinese can be: 为了子孙后代, 我们应该保护环境.
A direct translation of 我认为保护环境将为子孙后代保证未来 will be something close to:
I believe (我认为) protecting the environment (保护环境) will guarantee a future for offsprings (将为子孙后代保证未来)
While It's understandable, a better way of saying it could be:
1) 我认为 (I believe) was not in the origin content
2) 子孙后代 vs 下一代 - (your) offsprings vs future generations (which does not necessary related to you by blood)
its a bit unnatural translation, 将and 为 are kinda unnecessary together, not to say the direct translation is not wrong, everyone can understand what you are trying to present, but a native speaker would not phrase it that way. I think rephrasing it as 我认为保护环境会保证子孙后代的未来 will be more suitable.