You will normally be constrained in your choice of modules (if you have one) such that you have to select an amount adding up to an exact amount of credits appropriate for your study intensity (i.e. full time or part time).
Most UK universities use the CATS format (implicitly or explicitly) for credit values, which means you will normally take 120 credits of modules a year, adding up to 360 credits for a 3 year undergraduate degree. This dovetails the European Bologna process and ECTS credits system I believe, which is basically half as much (60 ECTS = 120 CATS).
Your personal tutor will advise you more on module choices if you have optional modules but in general you will need to take an amount equivalent to full time study (assuming a full time degree) which is like 120 credits total. As for the split between fall/spring term modules, this is up to the unis and how they teach and timetable them - some unis/courses organise it so most optional modules fall in the second term as you do most compulsory modules in the first term, although many have them evenly split across the year.
Basically don't worry about it too much - if your programme has optional modules in first year, make a note of any that sound appealing to you, and when you start you can discuss with your personal tutor which options you may be able to take. Also don't set your heart on any one option, as you may not know until later when the timetable is released if you can take it in case of clashes for example!