FIMing

when should i start it and how

you can fim at any time, and you just cut off about 3/4s of the growing tip.. i am sure there are lots of good turorials on here with pix that show you exactly how and where to cut to fim..

well its on 2nd node ready to go to 3rd so i should cut 80 off

and can i do fimming and topping on the same plant

fimming and topping are almost the same thing.. i would think that you could fim one time, and then later on, top her.. you can top as many times as you like.. if i were you, i would go and check out uncle bens topping technique for two or four main colas.. its a great thread on here, and he does a much better job of explaining when and where to top than i would do for you..
after i post this, i will find the link for you and post that too.. hope ub's post will help clear some things up for you, i know it has for me in the past..

https://www.rollitup.org/advanced-marijuana-cultivation/151706-uncle-bens-topping-technique-get.html
found it for you.. lots of great info on here and i think it will help you out a ton.. gl..

thanks alot

no offense to uncleben but i prefer this one,
https://www.rollitup.org/advanced-marijuana-cultivation/200413-fimming-m-blaze-complete-how.html
I would not fim till at least 4 nodes. otherwise your just gonna shock the plant when you fim it at first and then it will slowly start the new branches. if its bigger you have more leaves to absorb light and make energy to heal and take off growing again w/the new tops,