In natural spoken English, most one-syllable nouns, verbs, adjectives, and adverbs actually are stressed on their own (some verbs, like modal verbs, and versions of have and be very often aren't). Dictionaries don't mark the stress on them; because they only have one syllable, so whether you stress these words or not depends more on the meaning and the whole sentence structure rather than the words themselves.
On the other hand, function words like to and a usually aren't stressed on their own in English speech.
These rules are often broken, for example when people want to emphasize words, or when too many unstressed (or stressed) syllables come in succession.
Using these rules (which are definitely overly simplified) we would scan the lines as follows:
Shall I/ compáre/ thee to/ a súm/mers dáy?
Thou art/ more lóve/ly and/ more tém/perate.
Róugh wínds/ do sháke/ the dár/ling búds/ of Máy,
And súm/mer’s léase/ hath áll/ too shórt/ a dáte.
The stresses don't correspond exactly to the ones required by iambic pentameter, but they come pretty close. In the last two lines the only deviation is rough.
When reading poetry, some deviation from perfect iambic pentameter is expected. If you read this quatrain with the rather simplistic scansion above, in my opinion it sounds quite good (and better than it would if you mechanically stressed every other syllable), even though some literature professors would disagree with this scansion.
Literature professors have come up with all sorts of rules for how to scan English poetry, and not all of them agree. But the bottom line is that if the stresses in your poem are close enough to the stresses of perfect iambic pentameter, it will sound good.
