As Prosperity Park said there's a guide section for builds. Also check out
http://www.mechspecs.com/forum.php for additional builds.
Here's what I'm running right now:
1X, 340XL engine, 3x Large Lasers in arms, AC2 w/ 1 ton of ammo, rest of the space in DHS (don't forget that 3 can go in the engine!)
This mech cooks along at 86kph w/ Speed Tweek and is very heat efficient. AC2 ammo is light but I only really use it for suppression - real damage is from the lasers.
3D, 280XL engine w/ 1 JJ, 4 Medium Lasers, 2x UAC5 w/ 6 tons of ammo, ES and DHS
Brutal shock and awe mech. Use the UAC5's at range lightly until the real fight starts. Then barrel down on some unsuspecting support mechs. Dual UAC5 and precision laser fire can tear an enemy apart in a hurry. It's hard not to walk away with at least one kill with this mech.
4X, 245 engine, 2x AC10 w/ 4 tons of ammo, 2x MPL, DHS and ES
It's only ok right now - once AC10's get buffed I expect this will be brutal.