We cannot suggest the best way to monetize a soccer betting forum unless one of us here is your competitor. Also, to let us provide suggestions with better context, you should give us a range on the amount of traffic that your forum is receiving rather than labeling as "decent".
Anyway, judging from your replies, I guess walking you through the different types of ads monetization methods is best for now.
Publishers like yourself gets paid for advertisements and the rates are usually calculated in the following basis:
CPC (Cost-per-Click): You will be credited when a site visitor clicks on the ad. This is the most common form of ad rate calculation and it is adopted by most advertising networks.
CPM (Cost-per-Thousand Impressions/Views): You will be credited on a cost per one thousand impression basis. Impression is basically how many times an ad panel is loaded on your website. For example, if you have 3 ad banners on a page and the page is visited 1,000 times, then you will have 3,000 impressions.
CPA (Cost-per-Action/Acquisition/Lead): This is suggested if a large % of your forum members are active. CPA advertising models are usually adopted by affiliate networks rather than the usual advertising networks. They pay you a % which may range from 4% to 50% or a fixed commission when a visitor purchases or signs up for something after clicking on the banner on your forum.
If you have time to reach out to marketing executives or managers working with soccer betting services, you may even reach out to them to hammer an ad sponsorship deal. For beginners, the price that you are quoting for this forum sponsorship deal can be a fixed monthly amount.
I would also like to highlight that you may face complications with Google AdSense as the service places some restrictions on publishers that falls under the gambling/better category and different countries may have different sets of limitations. I found this AdSense help article that might be useful in helping you decide whether to join AdSense.