You've got a 2nd order equation, so that means you need a 2-element state vector. The two states will be y and y'. All of your code needs to be rewritten with a 2-element state vector [y;y'] instead of the single state y. I find it convenient to use a column vector for this. So everywhere in your code that you are using y, you will need to use a two element column vector instead. E.g., some changes like this:
y = zeros(2,numel(t)+1);
y(:,1) = [1;0];
f = @(t,y)[y(2);-1];
k1 = h*feval(f, t , y(:,m) );
Also, you have a bug in the line that combines the k's ... you need (1/6)* instead of (1/6)+
Make an effort at implementing these changes and then come back with any problems you continue to have.