When ever I play this map surf_ski_2_go I always see these guys jump, and when they do jump they get this amazing amount of speed just from jumping. I was wondering how could I do that?
What they are doing is called bhop. In order to do so, you have to jump and strafe while on the air and then repeat untill you get the speed you want.
In surf maps, you'll see a lot of people doing this but not just to gain more speed, but to prestrafe, this means having more speed than normal when starting the timer. It's not as simple as bhopping and landing on a ramp though and this technique is mostly used by pro surfers in order to beat their times.
You are unable to get such speed in normal games. What you are seeing is one of the main mechanics of Surf maps. These types of maps completely change the game by making obstacle course which require high amounts of precision and speed in order to pass certain parts of the stage.
These maps and servers work by increasing the value of sv_airaccelerate beyond the default value of 10 to around 150 or 1000 on some servers. This command changes how quickly a player can turn without losing speed. When playing these maps to gain speed on ramps you need to hold A or D plus holding W depending on which way the ramp is facing. This allows you to pick up speed in order to perform the tricks the map requires.
Source: Information on surfing (TF also uses the Source game engine.)