Topas is very good, yes, and since AIX 5.x it is automatically installed iirc.
Also a simple vmstat can show a lot of stuff at 1st glance.
As rhfrommn said, nmon is very good too and has tools to make Excel-Sheets or graph charts etc.
To interpret such data you have to read about performance tuning as in several tuning guides on the web or the IBM Red Books.
I also like the publications of Jaqui Lynch you can find easily on the web, as this here on her webpage:
http://www.circle4.com/papers/pseries-a26-aug06.pdf
Also her initial tuning recommendation is good too.
But don't look at tuning like you have some set levers that you pull and buttons you push and all get's better. There are sometimes big differences in how machines are getting used and often AIX can be tuned as well as possible and the application(s) still "spits in the soup", because it needs tuning accordingly.
Overall, experience is of course very important too, but there is a point where you have to start
And where people work, problems can occure, that's natural and so absolut normal... Oh, I got philiosphic lol