how to play at number 8

Often the number 8 is the tall powerful one of the pack, as a number 8 you should be making a lot of runs, run straight hard and low! you will often be making runs from the back of your scrum but you should also look to be making runs in open play either by supporting your team mate who off loads to you or doing crash ball.
You should train to get stronger and faster.

Pierre spies is the South African number 8, as you can see he make lots of powerful runs and breaks the line on numerous occasions, he is also extremely fast despite weighing 108 kg's!

scrums:
In the scrum the number 8 is located at the back pushing behind both of the seconds rows, your job in the scrum is to support the seconds rows by pushing them forward and keeping them tight by using your arms to pull them together.
If you win the ball in the scrum then when the ball comes back to your feet you need to use your feet to keep the ball in the scrum and relatively still for your scrumhalf.
If your near the oppositions try line then you have the option to take the ball, go blindside and run your self, when doing this its best to do it as soon as the ball comes to the back of the scrum as to surprise the flanker who if any good will be expecting you to do it, you also want to explode out of the blocks as fast and powerful as possible, if your not able to get to the try line then you can always draw there player, either the flanker or scrumhalf, and pass it to the player outside you.

Open play:
Similar to a flanker you should be trying to be one of the first to most of the rucks and securing the ball for your team, as well as that though if there's no need for you to go into a ruck either because the balls already won or lost then get the backs line!
Having an extra man in defense is also helpful and when your attacking there's a couple things you can do depending on you and your backs, you can firstly just join the backs line and act as a back, this is mostly fine because you should be strong runner with alright ball handling skills but if your backs like to run lots of moves then you can get in the way and then they get annoyed at you; you can also run lines of your backs so stand behind your player slightly on his inside, this can work well is your running  fast and straight and attacking a gap, people find harder to tackle on there inside shoulder, if you do this well it can be a great way to break the line; finally you can simply stand behind the line again and just support the player with the ball so when they get tackled your the first to the break down and securing the ball.
Read my guide on rucking here.

Line outs:
Often you'll be lifting or occasionally be lifted in the lineout, you can see my guide for that here.