Do I simply dial country code+phone number on the Skype phone pad on my iPhone4S and just press call?
How do I check remaining credits and minutes?
Yes, that is all you would have to do to dial a number from the dial pad in the Skype app.
Keep in mind that the dialing pad will usually start out with the country code for where your Skype account was set up in (i.e., +44 for the UK, +1 for USA/Canada, etc.), trying to be helpful. If that is the case, you could backspace over whatever is showing up there, leave the + sign, then dial the country code and the rest of the phone number including the area/city code (drop leading zeroes in the area/city code). For example, London's 020 area code becomes 20 (+44 20 xxxx xxxx), Sydney's 02 area code becomes 2 (+61 2 xxxx xxxx), etc.
You will be able to check on the amount of Skype Credit you have, along with the status of any Online Numbers and whether or not you have Skype Voicemail, from the "My Info" part of the Skype iPhone app.