Going in circles here

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Fitting the bike entails a number of changes on a bike that make sure you are in a comfortable position while you ride, and that the parts are commensurate with your size. This is different from just 'sizing' the bike, where you look at stand over height and eyeball the fit. Adjustments a fitter might make: seat height, perhaps swapping the stem to a shorter/longer size, making sure the width of the handlebars is appropriate for you (e.g., in a road bike, the bars are typically matched to your shoulder width -- however, I am perfectly comfortable with handlebars 4cm wider than my shoulders). Normally, your bike would be put on a trainer and you pedal a number of minutes to warm up and to see how you work on the bike. Some stores offer a basic fitting when you buy a bike. You can always pay for a fitting -- prices might range from, say $100 to $300, new parts are charged separately. [for example, when I was fitted, I spent about 3 hours with the fitter, who did an assessment of flexibility, range of motion of some joints, measured shoulder width, arm length, forearm length, inseam, etc... I pedaled for more than 20 min to warm up, the fitter put tape marks on my knees and filmed to see how may knees were tracking, we tried different pedals, tweaked saddle fore/aft, etc; I had been cycling for nearly 7 years when I did this]

Is it the right bike? Hard to know -- is it your size and does only need some minor adjustments? If it is the right size, you can always change the saddle for something that is a better fit (this forum has so many threads about saddles, including how to measure your sit bones, that your head will spin). You could also change the grips on the handlebar -- Ergon grips are very nice. The ones with bars give your options to make your hands comfortable during rides. Needless to say, adjusting the saddle height and making sure it is in the right position (saddles have rails and you can move it forward or backwards, make sure the saddle is level, etc... Two rules of thumb:

• Saddle height: at the bottom of the pedal stroke, your leg should be almost straight (ie, your knee should be short of "locking" in the stretched position)
• Saddle fore/aft adjustment: The rule I've seen applied many times is to have the pedal at the "3 o'clock" position. If you drop a plum line from your knee, it should be right above the ball of the foot (the foot on the pedal, of course). The idea is that you don't you knee forced beyond the ball of your foot or exerting force behind it, either...

Roadmaster bikes are notoriously heavy, so eventually you may want to go with something else. You can find very nice, complete bikes. You still may want to get a saddle that is comfortable for you, but do not have to build a bike from scratch. You can always replace some components to tweak something or the other --that's when the hobby comes in.

Tires: Many of us have a floor pump at home and are somewhat religious about making sure the tires have the right pressure before each ride (ok, I am not super religious... I just feel the tires before each ride), but, yes, tires lose some air and it is wise to keep them properly inflated to avoid pinch flats.

Happy riding! Biking is a lot of fun.