I present, in no specific order the top mistakes inexperienced bakers will make..

Forgetting an ingredient:

With the exception of very few recipes (fatless sponge for example) a cake will have flour, sugar, eggs, and butter (margarine or possibly oil - but some kind of fat). If you find that your recipe doesn't have one of these ingredients make sure that's correct. Something that can help is to lay out all the ingredients up front (and you get to pretend to be a TV chef too!)

Over or under cooking:

Set an alarm, and make sure you check the cake when it goes off. A counter point to that is don't keep opening the oven - especially for the first half of the cooking or you'll let all the heat out and the cake won't cook.

Skewers and prodding are your friends when trying to figure out if a cake is cooked.

Don't panic if you over cook - you can chop off the burned bits and carry on if there aren't too many.

If it's undercooked - not really much to be done except turn it into truffles (slice and dry out slightly in a very low oven so it'll soak up the flavouring)

Not waiting for the cake to cool:

Before turning out of the tin, before slicing the top off to flatten it, or for modelling just wait for the cake to cool. Warm cake is fragile and it will break

My cake didn't rise!

Was the oven up to temperature when you put it in? You need to wait for the oven to be warm (usually a little light by the temperature dial goes out)

 Did you continually open the oven to check if the cake was cooked? Don't open the oven for the first half of cooking, and after that only open it a maximum of once every 10 minutes

Did you leave the mixture sitting around too long? Cake mixture should be used within about an hour of being made - and preferably straight away

Did you use plain flour? If you used plain flour, you're going to need to add baking powder to it - baking powder is what makes a cake rise, and self-raising flour has it already added unlike plain flour