Ooh! I have just sent DSis a list of things she could try to make her cakes edible - she makes really good rock cakes from all sorts of recipes 
Get an oven thermometer and check your oven.
Pick one recipe - Victoria sponge is probably the best as it is very edible as a biscuit, as others have said, custard is a great cake saver. In this weather cream and chopped fruit works well too!
Delia's recipe for a Victoria sponge, and her all in one version, are easily found and work well.
Check your tin sizes, actually measure them, see what you really did buy.
Consider getting a packet of sponge flour, see if that makes a difference. About 15 years ago something changed in flour production. I had to modify a lot of recipes. For basic sponges the sponge flour could make a difference.
Don't use eggs from the fridge. They don't need to be kept in there anyway and the coldness of them can stiffen your mix.
Beating the sugar and eggs... that can be overdone, read Delia's instructions / description and don't believe anyone who tells you an extra 5 minutes give a better result - it doesn't, it 'cooks' the egg protein and can inhibit the rise, make the crumb very dense.
Make sure you know the difference between beating or whisking and folding. I still use a large metal spoon for folding, out of habit as I have a folding spoon my great grandma used, flat on one edge. But be sure you know how to fold and don't rush it.
Also stop and consider 'dropping consistency' - Delia explains. It is worth getting this right as it forms the basis of a lot of cake mixes.
Don't push the cake mix into the cake tin, it barely needs smoothing, don't bang it to level it. Some cakes need a dipped or domed finish, but they come later 
Pop it into the oven, I don't use a middle shelf, too tricky to get to to test! 5 minutes before end of cooking time test it: does the cake rebound when you gently push it? Is the cake coming away from the cake tin - small gap round the edges? Does a skewer come out clean?
Use all 3 methods, you'll fund the one you like. You can work the timing back and forth until your cake is just cooked, not dry, not soggy.
Once you have a perfect Vicky sponge you will automatically know what makes a good cake batter and all your other cakes will get better - I promise!