How can you get more horsepower out of your 2004 Dodge Dakota sxt 3.7 v6?

Depends how much you want to spend. You can probably get a power chip for it, but they can be pricey. Likewise, having intake and exhaust manifolds ground out inside (removing certain ridges and casting flashings can improve airflow for increased hp)(not a DIY job for the inexperienced) can be pricey and results may be too small to be noticeable. Installing exhaust headers, and/or increasing exhaust pipe diameter may add power, but will likely affect emission controls adversely as will removing the catalytic converter (removing the catalytic converter is also illegal in most or all US states and Canadian provinces). Installing a freer flowing muffler may add some ponies as well but will likely be louder.

For a "cheap", although typically not a very noticeable power increase, do a tuneup, an oilchange, and replace plugs and wires if they are older, and most especially replace the air filter. Avoid using power hungry options like a/c (a/c on with the engine fan also on, robs about 30HP!!).

For a noticeable increase in power, replace the air filter with a K&N oiled media, non-disposeable, washable filter, but beware; most manufacturers don't recommend this since the filtration is not as good. It won't affect your warranty except if engine failure can be attributed to poor intake air filtration (which the warranter is guaranteed to try to blame if possible).