Cyclisation of acetone on reaction with conc. H2SO4
You may have noticed that your question received a downvote. This was probably because you did not show any effort toward a solution of the question. I posted in a Comment some clues to assist you in a solution. In the interim an Answer was posted that showed the compounds formed in the reaction of acetone with H2SO4 but it did not illustrate a mechanism for the formation of mesitylene, which was your question. Here is my contribution to your understanding.
The reaction between the keto form of acetone 1a and its enol 1b forms aldol 2. Acid-catalyzed dehydration of 2 via the enol 3 leads to mesityl oxide 4. The loss of water from 3 may be stepwise but, to save space, I have presented the loss of water in a single operation. The third unit of acetone is incorporated via the vinylogous enol 4b to form the dienone 5a. Enolization of 5a forms 5b' that allows rotation about the red single bond in 5a and 5b' to form conformation 5b". This species may cyclize by an allowed thermal electrocyclization to form cyclohexadiene 8 which is readily dehydrated by acid to mesitylene 9. Alternatively, enol 5b' may reketonize to 6 and undergo cyclization to cation 7 and thence on to mesitylene 9. continued...

Addendum: An old name for acetone was mesite, the source of mesitylene. Read about it here and here.