Firstly, while it may be easier to call them 'accents', they are separate letters with separate pronunciation.
ż (Z with a dot) is roughly English zh (which of course is not a common sound).
ç (C-cedilla) is not a Polish letter, it's Portuguese (and some other languages).
ć is probably what you meant. I'd say that it's a palatalized T. The palatalized sounds are very difficult for Polish learners.
ę is roughly 'eu'. Imagine the Spanish pronunciation of "Europa".
ą is roughly 'ou'. The vowel sound in "rose".
If you say "XYZ, please", "proszę" sounds a lot more natural to me.
"Poproszę XYZ" is more like "I'd like to ask for XYZ", but really it just means the same as "XYZ, please" and I think it's more polite.
Technically speaking, "poproszę" is future tense (I will ask), but it's used for actual asking 'right now' ;)
Jellei explained the difference between “proszę” and “poproszę” very well.
It is also good to note that “poproszę” works only when you asking for something, while “proszę” may be used both ways: asking for something and offering something.
When you put a cup of tea in front of your guest you say “proszę”.
But still more common way to ask for something is by using "poproszę"
Please check out 1:18 in this video and notice how "poproszę" i "proszę" are being used:
It's not a cedilla, although it's similar. Even in English it's called "ogonek", like in Polish, which means "little tail".
It's a nasalized "e". It's more or less like "eu" in Spanish (euro, Europa), not sure if there's a similar sound in English. But that happens if "ę" is in the middle of a word. If it's the final sound of the word, it's pronounced more or less like a normal "e". And if it's the first sound of the word, it's not a real word ;)
Proszę o chleb i mleko - I am asking/ I ask for bread and milk
Chleb i mleko, proszę - (May I have) Bread and milk, please?
Chleb i mleko, proszę - Here you are, your bread and milk
Proszę (bardzo) - You are (very) welcome
Poproszę o chleb i mleko - I will/ I am going to ask for bread and milk