New "Rocket Fish"???

Ok so im new to surfing for the most part. I have ridden my friends shortboard and been able to stand with no problem. So I went to the local surf shop and bought a used fish. I used it today in 3 foot waves or so and found it much more difficult to stand than it was on my friends shortboard. I was able to stand but i had to keep my body low if i straightened out i would fall. My board is 6'2" looks like a shortboard but just is a little wider and it has the swallow tail w shape and a hip about a foot before the tip of the tail making is skinner than the rest of the board. Umm.... its also a twin fin. Sorry for the horrible description like i said im new. So is there anything i should know about this board and any tips to help me get up and going??