Hi everyone I'm only a beginner with making cakes I've not done anything for a while now and really wanted to do 2 tier birthday cake .No sure what size cake tins to use can anyone offer advice pls.

For how many people?

8 people

http://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm

Hope that helps :)

Well, the Wilton Cake Baking and Serving Guide shows an 4 inch in diameter, 2 layer (two 2 inch layers) round cake serves 8 party sized pieces.  An 8 inch in diameter, 2 layer, two inch deep layers, round or square is shown to serve 20.  Those are fairly small sized pieces by my standards.

To get a nice look, I would likely make an 8 inch in diameter either 3 inch deep cake (torted and filled or not torted and  not filled) or the 2 layer, two inch deep filled, 8 inch in diameter cake.  For the top tier I would make either a 4 inch in diameter 2 inch layer cake,  filled or 4 inches in diameter, 3 inch deep cake torted and filled or not torted.  For a nice appearance you need at least 4 inches in diameter difference between the two tiers.  This will give you 2 inches all around if you center your top tier over the bottom tier.

Of course, you are going to have lots of leftover cake.

 Your top tier should be on a supported cake board or plate for ease of serving.