So, you've got 3-bromocyclohexene, and you want 3-methyl cychohexa-1,3-diene. Looking at that immediately, I see a few things. You've already got one of the double bonds you need, and the other double bond you want is in a pretty convenient position. You also want to add a carbon to where you already have a bromine. So, first point of info you should have - if you've got one functional group somewhere you can usually mess with it until you get another functional group. Similarly there are ways to go between functional groups and double bonds.
So find your textbook, and see what functional group to double bond reactions you know. See what carbon addition reactions you know (my first instinct is to look at that bromine and immediately think Grignard, but I honestly couldn't tell you without busting out my own textbook). Look at your tools, figure it out.
Still, to even do that there are things you need to know. You need to be able to look at a problem and identify opportunities (that bromine right there in the reactant, right by both your double bond and the methyl group you need), and you need enough idea of how your tools work to know what to look for.