Susana - from Santo Domingo, Dom. Rep.
There are a number of egg-free egg substitutes available on the market that can replace eggs. There is also egg free cake mixes available, check at your local health food store or the healthful food aisle at your local grocery store.
Vegetable shortening can be used as a substitute for butter or margarine. These products will give a similar but not quite exact texture as butter. Vegetable shortening now comes in butter flavor. Try to choose shortening that is trans-fat free. Do not substitute liquid oils for butter or margarine as they will completely change the texture of the recipe you are making.