Molesworth, please do yourself a favour and read the manual from cover to cover. But to get back to your question, here's the powersetting table from a C152 POH (check if it matches your aircraft before using!):
If you're going cross-country find the row for the pressure altitude that you'll be flying at and the column for the temperature. Then choose a %power from the '%BHP' column and use the numbers to the right of that. Set that RPM when settled in the cruise and keep it at that number. Keep in mind that you'll need the engine at 'Recommended lean' i.a.w. POH chapter four to get the fuel flow numbers mentioned here, and even then add a percentage to your trip fuel for safety's sake (5-7% would be my recommendation but more fuel is always better as long as you're within the W&B envelope).
Example: For 2000 feet on a standard day, 66% gives you 96 KTAS at 2300 rpm.