Buttercream - A good rule to work by is the half fat to icing sugar ratio (confectioners sugar for the US). It's usually known as the half fat to flour ratio, but I didn't want to confuse you, lol!
Therefore for every 100 grams of butter, you would use 200 grams of icing sugar, and for 300 grams of butter, you would use 600 grams of icing sugar etc. For best results, always use a block of butter as this holds its shape and produces better results when piping flowers etc. Also, see Buttercream Tutorial for full details of how to make it.
Filling - Always be very careful with the amount of filling you place in a cake; less is always more. If you add too much filling, you can get a sliding effect, so when you come to cover the cake, the top section will slide when you are trying to cover. In order to prevent this, either slice your cake into additional layers so a slither of your filling can be placed multiple times within the cake or simply add a little buttercream and a little jam.
Icing - Whether it's called icing, fondant sugarpaste, regal icing, decor ice or any other name on the planet, this is simply icing that can be rolled out and used to cover a cake. It comes in a multitude of colours and can also be used to make models and edible decorations.
Royal Icing - This is used for piping, decorations and was applied to old school wedding cakes. It was built up in layers numerous times until it was thick enough to cover the cake, it also sets rock solid, so it's great for sticking decorations to your cakes!
Supermarket brands vs Cake shop-bought icing - Depending on what you are using, the icing for will depend on which brand you will use.
Supermarket Icing - You can buy blocks of ready rolled white icing from any supermarket these days, and there is usually an array of goodies down the cake aisle. I have found that normal white supermarket icing can be great for learning with; it is easy to dye and can give you great results. It's also relatively good on the bank balance and can be a great playtime with you and the kids, some supermarkets also do their own coloured icing range, but I don't use them. If you want instant colours that the kids can make stuff with, then yes, buy them, but if you are making a cake for someone, I would advise either dying the supermarket brands white icing to give you the required colour or buying a cake shop brand.
Cake Shop Icing - I must admit that I tend to use Renshaw Decor Ice when making cakes for people, which comes in multiple colours. It is very easy to work with, gives you great results and tastes good too. I have used other cake shop brands but found that Renshaws works best for me. You can buy this online direct from The Vanilla Valley. Also, Amazon sells the icing from multiple sellers.
Concentrated Dyes - When dyeing white icing or buttercream etc. I also favour the Sugarflair range of concentrated dyes. They are in small tubs, come in a rainbow of colours and are pretty much free from all allergens, which is another factor cake makers need to be aware of. You only need the tiniest amount to create your desired colour, and they last quite a few years too. The only colours I tend to avoid making is deep colours such as navy blue, black, dark green and red. The reason being is that sometimes a large amount of dye is required for you to obtain the results you require, the problem with that is too much dye can alter the consistency of your icing and make it bitter, and like chewing gum or with your buttercream, it may cause the mix to split or curdle. To avoid this problem and much wasted time and effort, I opt straight for the Renshaws brand.
Sponge Cakes - If you are making a single-tiered cake, then a sponge cake can be a perfect choice as it can be made into many flavours and are extremely light and fluffy. They are also good if you are adding very light decorations to your cake, as heavy decorations on a sponge cake can cause a dip. If you use the highlighted recipe and guide rather than splitting the mix into sandwich tins, it can be poured directly into a deep tin and baked for approx 50-55 mins in a 6-inch deep tin on 150°C. This recipe can also be doubled up.
Carved Cakes - When making a carved cake, it is always better to use a sturdy cake such as a Pound cake or Madeira cake. These are better to carve out shapes with and give you better results than regular sponges. A great book with carved cakes for all occasions is Debbie Brown's 50 Easy Party Cakes; she covers numerous celebrations and gives you fun and creative cakes. They have a step by step instructions, and you'll be surprised at just what you'll be able to make. However, if you are looking to hone your skills but on a smaller scale, then try the Ann Pickard range of books; they have some lovely designs and are very simple and fun to make.