I wouldn't recommend sector investing. Invest in total market and take the market weight of each sector.
If you plan on buying ETFs and maxing your Roth IRA, you should be able to have a fully diversified asset allocation.
For example, my ETF portfolio is:
35% VTI
35% VXF
15% VEU
15% VSS
I'm 100% stock with a tilt towards small and mid-cap.
So in my opinion, develop a plan and pick the asset allocation you want for long term. Then go ahead and buy ETFs according to that plan.
A simple 3-fund portfolio might be a good place to start:
http://www.bogleheads.org/wiki/Three-fund_portfolio63% VTI (total US stock)
27% VXUS (total international stock)
20% BND (total US bond)
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