This is the tool I would start with if I were trying to test an online idea from scratch.
Systeme.io has a permanent free plan and does not require a credit card to open it. The free plan currently gives you up to 2,000 contacts, 3 sales funnels, 1 course, 1 affiliate program, 1 custom domain, and unlimited email sending.
That is plenty to build a simple landing page, collect emails, send follow-up, and find out whether your idea is worth taking further.
I would stay on the free plan until you actually hit a limit that is getting in your way. If you eventually decide to upgrade, Systeme.io currently offers two months free when you choose annual billing.
Referral disclosure: The button above is my referral link. If you later buy a paid plan through it, I may earn a commission. Your price is not increased because of my referral.
Use the free stuff until the free stuff stops doing the job.