On the physical front, softer and more sensitive skin is generally the norm, but as with real life, there's probably a high degree of variability.

The most noticable change will obviously be the various aspects of the different body shape: centre of gravity, weight distribution, breasts, the change in equipment 'down below' (which of course involves a different procedure when visiting the toilet); possibly wider hips / spaced legs. Usually this coincides with a dramatic increase in hair length (it's fairly common for the boy's close crop to grow out to shoulder length or below, and typically they're prevented from cutting it back [if at all], not even to a bob or pixie).

The next most obvious change will be in clothing - most of their previous clothing will no longer fit; underwear will obviously be different (especially with the addition of a brassiere!), outer clothing will be styled and coloured differently. If the story's taking place in modern day Western society, they can probably get away without crossing the hurdle of tights (pantyhose) / skirts / dresses (although some schools that have unifroms still don't allow girls the option of wearing trousers)... unless it's a bodyswap type transformation (i.e. where they find the girl they've become previously existed, whereby it's common for them to have to adopt the girl's mannerisms / behaviour / dress sense to avoid being discovered).

On the mental / emotional level, the world's your oyster. Sometimes it's purely a physical transformation (so they're left to cope on their own), sometimes it's accompanied by a built-in sense of acceptance, sometimes (particularly in body swap transformations) they find the body comes with an 'auto-pilot' (i.e. many of the autonomous actions, e.g. getting showered / dressed, are built-in and they'll do them automatically as long as they don't think about it; if/when they do, that's when the panic sets in).

For many, their first instinct will be "How do I reverse this?"; even if/when they realise it's permanent, some will fight it all the way, some will 'go with the flow' and do their best to adapt; some will take the proverbial plunge into the deep end (even if they never had T* feelings before, they feel the best way to adapt is to dive in head first, believing the more overboard they go, the quicker they'll adapt. Which can have varying outcomes...)

Once all that's dealt with, there's the tricky issue of relationships - both platonic and romantic. If they find their new body is androphilic, they may struggle to reconcile their mind (still set in male gynephilic patterns) with their body's desire. If they find their new body is gynephilic (i.e. lesbian), there's the whole issue of the extent to which they need to suppress it. In a fairly liberal setting in the modern day, that may not be such a problem. If they're in a very traditionalist setting, society may force them to seek out a male companion, even if they detest the idea. Needless to say, the ultimate horror for someone transformed as part of a revenge plot is to find themselves in the body of a prostitute.