Huge number of threads here on looking for an agent. Basically, interview at least 3. Look for knowledge, but also look for compatibility with you--someone who'll listen to you, respond to you . . . and respond in a way that you feel comfortable.
How do you make that specific offer? First, get an agent. Second, have the agent run a CMA (competitive market analysis) on the land. So, in that regard, make sure that your agent has experience with land, land sales, and land valuation. Once you get the CMA, you'll know approximately how much the land is worth. But land is tricky. What are your plans for the land? Just to hold onto and perhaps build in the future? Build now? Subdivide? Rezone and resell? All these really affect the value.
But once you know the approximate value to you, then you make an offer. Pay no more than the CMA. Probably offer less. Your agent can advise you on the particular strategy.
Hope that helps.