You must remember this, an 's' is just an 's',an 'i' is just an 'i' . . .

Some things never change and, thankfully, when learning Spanish, it's useful to know that one of life's constants is the pronunciation of the Spanish alphabet.
Unlike English, each letter has one sound and, irrespective of where it appears in a word, that does not change.
I'm not even going to attempt to give examples, as the words I might choose to illustrate a particular sound might be pronounced quite differently by a Southerner, a Scot, or someone from Liverpool, Birmingham, or Dublin.
Suffice it to say that, unlike a poor European, trying to determine how to pronounce 'cough', 'rough','through','bough' or 'though', once you have mastered the pronunciation of the letters, you will be able to pronounce any Spanish word, just by reading it. Anyone remember 'Catchprase'? Just 'say what you see!'

There are a couple of extra letters thrown in, just to make things a little more exotic, like 'll' and 'ñ' but they, too only have one sound each.

Obviously, there are regional differences, as there are in the UK, but they won't affect the way you speak, just the way you listen.
Notably, in northern Spain, 'c', in the middle of a word, before a vowel, tends to be pronounced as 'th' e.g. 'Barthelona' for Barcelona.
In parts of South America you may hear 'll' and 'y' pronounced as a slurred 'j', so 'yo llegué' (I arrived) becomes 'zjo zjegué'.
There is also a tendency, in Argentina for example, for the letter 's' to morph into a barely perceptible, nasal 'h', or disappear altogether, so 'lo mismo' (the same) becomes 'lo mihmo', and 'los lobos' (the wolves) are reduced to 'lo lobo'.

Don't worry about it, at this stage. Your initial aim is to be able to pronounce words clearly and be understandable.

If you'd like to experiment with some Spanish words, and find out how they will sound, here's a great text-to-speech site, which will read your input and speak to you with correct Spanish pronunciation.
http://www.oddcast.com/home/demos/tts/tts_example.php
Not only does it include a range of langages but also, at least in the case of Spanish, a variety of voices and regional differences.
Try typing 'lo mismo', for instance, and listen to the difference between voices of Rosa (Lat. Am) and  Francisca (Chilean), or listen how Violeta and Carmen differ with 'yo te llamo' (I [will] call you)
Apart from the multitude of voices, the application also recognises the extra Spanish characters, like ' ñ ', so it pronounces 'manana' differently to 'mañana'.

Obviously that's not a deal of use to you, if you can't type the special characters.
'LetslearntospeakSpanish' to the rescue! Just click over http://www.onehourprogramming.com/spanish-accents/ and download the brilliant little App which uses your CapsLock key as a 'shift' key to type Spanish characters like ñ á é í ó , and it's free! If you're submitting any typewritten classwork, it's a lifesaver.
Almost forgot, it also does the inverted exclamation and question marks.
¿Muy bien, no?  ¡Saludos!