For most people outside Birmingham, and even many in Birmingham, using the University rail station is the best way to get here. This is called 'University' (you don't need to tell the person in the ticket office which university - the railway system knows there is only one that matters - but by all means say 'University of Birmingham' if you want! This station is two stops out from Birmingham New Street station, on the line south to Longbridge and Reddich. Buy your ticket at the first possible opportunity to 'University' as it is the same price as to any central Birmingham station, but if you buy it in New Street you will have to pay extra.
If you are staying for a meal with us, you might want to buy your ticket to Selly Oak. This is three stops out on the same line, but is more convenient for the restaurants. That way you can get off at University and return from Selly Oak.
When you walk out of the university station (you are on the bridge over the railway line) turn left towards the university. Walk dead straight for 10 minutes. You go past an ugly modern statue of Faraday, but keep going, always straight in the same direction. The bookshop is about halfway on your left. After passing that you go under a bridge thing with offices. From the other side of the bridge you see the clock tower on the right and library on the left. (There's a nice Hepworth sculpture near the library you should be able to see here.) You continue dead straight as before to the next bridge thing with offices. Just before it you go up the steps you see to the left, enter the building here (it should say "Watson" and there are push-button doors for disabled people that never seem to work properly) and maths is on the second floor. We meet in 221 or 222 on the second floor but Richard Kaye's office is 308 on the third if it helps.
Buses go down the Bristol Road (A38) frequently and conveniently. Pay exact fare to the bus driver.
Maps and directions are at http://www.birmingham.ac.uk/contact/directions/index.aspx. The Watson building is R15 (red) on the main PDF map at http://www.birmingham.ac.uk/Documents/university/edgbaston-map.pdf.