Your question involves two different things:


1. Orthography.

When used after a consonant, letters я, е, ё, ю indicate the softness of the previous consonant:

тема — soft /t/ + /e/
тяга — soft /t/ + /a/
тётя — soft /t/ + /o/
тюль — soft /t/ + /u/

When used at the begining of a word or after a vowel, they get additional /y/ instead of missing consonant:

есть, заехать - /y/ + /e/
яблоко — /y/ + /a/
ёж, приём — /y/ + /o/
юла, краюха — /y/ + /u/

2. Vowel reduction.

Unstressed vowels undergo vowel reduction:
Unstressed /e/ merges with /i/: леса ("forests") and лиса ("fox") are pronounced in the same way with short /i/.
Unstressed /o/ and /a/ merge after hard consonant: валы ("bulwarks") and волы ("oxen"), both with /a/.
Unstressed /a/ merges with /i/ after soft consonant: пятёрка ("five") pronounced as if it were питёрка. Usually, this merge happens only in syllables before the stressed one (пятёрка, рядовой, тягать), and does not happen after it (тётя, котя, статуя, туя).
Unstressed /u/ also tends to merge with /i/ after soft consonant: тюбетейка is more like тибитейка, however some people pronounce /u/. This merge happens only in syllables before the stressed one.