When would I use either -ę or -m in Polish?

I am learning polish and I would say I'm at a high b1 to low b2 level, but I have always wondered for words that are talking about myself that end in -m, like for example:

Rozumiem
Czytam
Jem

When would I use the -m ending v.s. the ę ending, for words like

lubię
Idę

If I were given a verb in a dictionary, which ending would I chose?

There are different verb classes in Polish, have a look at this link and see if it helps you :-

polish-dictionary.com/polish-verb-classes

You could also get yourself a copy of 301 Polish Verbs by Klara Janecki which is a very good reference book.
It won't take long before you see the patterns of verb endings, and it gives verbs in present, past, future, conditional tenses etc.

Nice link, Chemikiem!

I've known several bilingual Polish Ukrainians who've sworn up and down that it's "Ja rozumię" vs. (CORRECT) "Ja rozumiEM"

:-)

The foreign-born learner such as I must learn to wade through dialect-induced error interference, even among certain native Polish speakers, in order to learn and retain proper Polish.

I just plod on with my books Lyzko ;), although I need some new material now....

@Tomcyzk, you could also have a look at this thread :-

https://polishforums.com/language/iec-conjugation-41259/

There is lots of information in the Language forum, so use the search function.

I have no idea if it's the case and if this rule would work always, but look at the infinitives:

rozumieć
czytać
jeść

but:

lubić
iść

If there is a vowel before the cosonant(s) in the infinitive suffix, then you use "-em". Otherwise "-ę".

Verb classes in Polish are to be sure a challenge, particularly those with "exceptions" which would seem to depart from the so-called rules, for example "znać" (to know a person or place) - first person "Ja znam" vs. "(za-)poznać" (to meet, recognize) - first person "Ja znaJĘ" etc.

Here, it's often difficult especially for a first-time out learner to figure out whether the rule departs from the exception or vice-versa!

Whoops, I meant "poznaJĘ":-)

What a blooper!

Oh no! your image of Polish language guru collapsed :-/

He-he! Funny, Wulkan.

Zapoznać means to tell each of two people that are your friends, but they don't know each other, and they meet by chance, the name of the other one - however it's called in English.

Zapoznać się (z) - get to know (with). With a definition, with a task, with a text, with a new job, with new equipement... Rather not with a person. With a person it's poznać się.

"To become acquainted with" would be the customary translation.

"Zapoznać" would be rather "acquainte somebody with somebody", if I can say so in English.

For the first person singular which is what the OP is discussing, I think this would be right a lot of the time.
But for the verb Czytać which you mentioned, the ending would be 'am', Ja czytam, so not '-em' or '-ę', or have I misunderstood you somewhere?

I think it would be worthwhile for the OP to try and understand the classes though, as endings change less reliably with 2nd, 3rd person singular etc.