Improve erg time for 80kg 183cm 18year old.

[youngfula]

I am currently starting my pre season training schedule with the ambition of making the national team in either sweep or sculling. i have just come back into rowing after a 2 month holiday and am really feeling unfit. my best erg last year was in the region of 6.38 and am wanting to break through the 6.20 barrier this season i was woundering what the best training regime would be for me to get down that low and what should focus on weight training? erg training? or in the boat?
aswell as what type of SPM i should keep through the 2km erg peice as i have seen similar sized guys hold 36 for the distance? how would i be able to do train to hold that high on an erg. any help is much appreciated