There are hundreds of piping tips out there to chose from, you could buy them all (which is something I'm always tempted to do, with any baking or craft product!) but in reality you will only need a handful to be able to create gorgeous cupcakes and lambeth cake designs. 
I have chosen two sets for you to chose from, this one - a set of 6 stainless steel nozzles - will let you create lots of beautiful buttercream designs.
 
Included are tips:
1M - The most popular tip for your buttercream swirls.
2D - Again, very popular, used for creating buttercream roses.
4B - Perfect for creating small meringue-like open star patterns.
D66 - For larger open star patterns and swirls.
1A - Round piping tip which also doubles as a circular fondant cutter. I often use this for cutting "eyes" or "wheels" out of fondant.
234 - I call this the grass effect tip, perfect for creating texture of grass or "fur" on your buttercream animal or garden themed cakes.
 
 
Dishwasher safe. Made in China.