Depends on your location.
If in an urban area, your best bet is someplace that doesn't get heavy foot traffic. Look for areas that people only rarely go, such as warehouse districts. Alternately, consider quickly renting a storage unit and leaving the body there. Pay in cash, and wear something that obscures your features: this will give you a couple days before it begins to smell to leave town. Avoid anywhere public or where decomposition may increase the chances of finding the body: parks, rooftops, and basements are all no-gos. Dumping into a nearby river or body of water may work, provided you seal the body in something that will prevent decomposition from letting pieces float up where they can be seen(for a quick fix, a larger cooler sealed with duct tape can easily accommodate an average-sized adult).
If in a more rural environment, options increase. Burial is a good option, particularly if one has access to digging equipment: bury in a hole at least 8 feet deep to prevent easy discovery. Cremation can be useful if one can provide a hot enough flame, though bones will likely need to be disposed of separately unless you have access to an incredibly intense propellant. Wood chippers are also a good option if you need fertilizer, as decomposing flesh provides lots of nitrogen for plants.
...what?
