There are two aspects to your question:
- Are the chances of getting a visa for Germany for an Iranian citizen good?
- Are the chances of you specifically getting a visa good?
For the first question, you'll find detailed statistics on this page. The latest stats are for 2022.
The relevant data is:
Where you see that the rejection rate for applications for Germany from Iran is about double that of the worldwide average.
Not the worst in the world (many combinations are well over 50%, even when filtering out those with very few applications), but not a walk in the park either, you clearly start with a handicap.
Interestingly, the proportion of issued multiple-entry visas is extremely high, so this means they mostly issue visas to people who have either a long history of travel to Schengen, or people with very strong applications.
For the second question, the thing you have to remember is that when you apply for a visa, the main thing they try to determine is whether you are likely to respect the terms of your visa, mainly return home in time (and not overstay, not claim asylum, not work or do any other activities prohibited on a visitor visa, etc.). This mainly means you need to show ties to your home country.
And there we see a number of issues:
- You are single. Not an issue per se, of course! But that means you don't have a family to return to, kids to take care of, etc, so you can't use it as "ties to your home country".
- You work online, so you could probably do this anywhere in the world, including in Germany, and do not need to go home for this. Again, difficult to use as "ties to your home country".
- You have friends in Germany, which could help you settle and possibly even try to "stay under the radar".
- It's your first travel outside of the country (and of course to Schengen), so they can't judge how much you abide by visa rules.
So you have very little motivation to go back, and this can be a problem. It would help if you had (real, not invented) other ties, such as owning property there for instance, or having to take care of a relative (but the fact you can leave for a full month probably invalidates this).
Also, you say you want to visit Germany for a month to get a CELTA certificate. I'm not familiar with CELTA, but from what I gather, taking the exam should not take a month, and there are a lot of other locations you could take it, so they are bound to ask: why Germany? You could take it in Türkiye for instance, where you don't need a visa, which is closer to Iran, and where the cost of living is lower.
Talking about costs, you'll also have to show that the cost of the trip (flights and other transport + accommodation + meals + any activities you intend to perform during your visit) is proportionate to your revenue/savings.
Add to that that it's your first application, and that you are asking for a relatively long visa (one month), and my hunch (but that's only my hunch, I'm not a visa officer) is that your chances are quite low.
Curious to see what others think.