Community events are given to you via a random selection from Codemasters themselves, so you will probably need to wait until one comes up. However, there is usually a Delta Event most resets.
You can find these events in the Competitive section from the Main Menu > Community Events. Delta Event will be listed on the picture; if one isn’t there, then you will have to wait until the timer resets.
Getting into the Top Tier isn’t exactly hard, just play it cool and try not to crash or wipeout. You can check out the threshold by going into the current standings (this will always be the same) meaning if you land in Tier 1 when the event resets, you will get this trophy along with
The The Day Today achievement in DiRT 4 worth 52 pointsFinish in the Second Tier or higher in a Daily in Community Events
Once the timer resets, you will gain the trophy upon entering the community events screen.