First, learn how to write a function.
fun = @(x,y) x.^2 / 25 + y.^2 / 9 - 1;
Note the use of the .^ operators. They will be important in your use of MATLAB.
Next, learn how to use a tool like fimplicit. (Read the help for it. I won't do everything for you.)
Finally, learn why
wil be important. If you don't use that, then what is drawn won't look like the ellipse you might expect. See what the figure looks like before you call axis, then look at it after the call.
But really, if you have no idea how to use MATLAB, then READ the getting started tutorials.