Creating The BodyEdit
Open up your Blender, and delete your initial cube by selecting it with the right key and hitting delete. Now select the 'Add' tab top screen, browse to 'Mesh', and then 'UV Sphere', which creates a new sphere at the current cursor position. Change the default number of segments and rings to 16, and select OK. This should create a simple low res sphere, which will be stretched into the body of the fish. Ensure the sphere is selected, and change to edit mode. Select half of the sphere by pushing 'B', which allows you to drag a square around the elements you want to edit. Select the edges directly in the middle of the sphere, and extrude those edges to create a new face by pushing E, and align it to the axis parallel to the half removed by pushing X. Extrude by approximately 2 'Blends' (the unit of measurement in Blender, which can be seen when you are adjusting them in the bottom left). When you are done, you should have an image similar to this one.
With the edge still selected, extrude this edge by 1 blend, the same way as before, and scale again by 1.2, which should give you an image like this one.
Next to make the last half of the body, extrude again by 1.5, scale by 0.8 (reducing the size of that edge). Extrude by 0.5, scale by 0.7. Finally to finish the main body, extrude again by 0.25, and with the same edges still selected, push space, select edit, then vertices, and select 'merge' and 'at centre', which combines the currently selected edge into one vertice. This should produce the following image.
To make the body into a more fish like ellipse, select the whole model, and scale on the Z axis by 1.25.
Creating The TailEdit
Reenter object mode, and push space to create a new cube. Reenter edit mode with that cube selected, and create a new set of vertives on the model by selecting all of the cube, pushing space, going to 'Edit'/'Edges'/'Loop Subdivide'. A red line will appear which allows you to select where you want the cuts. Select across from left to right, and push '7' to create 7 cuts.
Select all of the edges on the right, and move them to the right (by pushing 'G') and move it to the right by 1 blend. Then as above, push space, and select 'Edit'/'Vertices'/'Merge' and then select centre.
Reduce the size of the tail on the Y axis. Then select evevy other line on the end, and reduce it again.
To finish it off, we will apply a subsurf modifier. What this will do, for each level that it is applied, will double the number of faces on the image, smoothing the surfaces. By using it, it will create a more smooth and organic model. First ensure that the editing panel is selected.
Then select 'Add Modifier' under the modifiers tab, and select 'SubSurf'. By increasing the 'Levels'/'Render Levels' options, the image will be made smoother, with the below image when it is set to two.
For the sake of this tutorial, always ensure that the 'Render Level' is set to the same rate as the 'Level'. This will ensure that the outputted model in the game will look identical to the model in Blender.
Using more or less the same techniques, and starting from a cube with a couple more loop cuts, you can easily make the head and side fins.
For the Side fin, create one and duplicate it with Shift-D, move it to the other side, and rotate it by selecting the duplicate and rotate it with R. This should create a pair of fins as below.
For the eyes start with a UVsphere, as with the original body, with 24 segments and rings. Duplicate it the same way as above.
Finally, for the mouth, create and scale a cube to form a thin rectangle, and place one loop cut in it as below.
Finally, place all the pieces together to make up a complete fish.