Lines 1-8 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Earth has not anything to show more fair:

While crossing over the Westminster Bridge, the speaker makes a bold statement: he has found the most beautiful scene on the planet. All you other artists can call off the search! Wordsworth has located the very heart of beauty, or "fairness."
Of course, though, he's exaggerating. He really means something like, "At this particular moment, I can't imagine anywhere being more beautiful than the place I'm standing." It's almost more a reflection of his mood than of the outside world. He can't compare the scene from the bridge with anything except his own memories, but since that's all anyone can do we'll let him run with this one.
The line ends with a colon, letting us know that he's going to tell us what earth is "showing" after the line break.

Dull would he be of soul who could pass by
A sight so touching in its majesty:

Instead of trying to describe the scene, as we might expect by now (hurry up, a sonnet is only 14 lines long!), the speaker tries to express how beautiful it is from another angle as well.
He justifies his decision to stop his coach along the way to look at the view from the bridge.
He says that anyone who didn't stop, who just passed by with a glance, would be "dull...of soul." The opposite of dull is sharp, so we're imagining that the speaker's soul must be like one of those knives they advertise on TV that can cut through coins.
The person who could just pass by has been jaded and worn down by experience to the point of dullness. He's also boring, which is another meaning of the word "dull."
The sight from the bridge is "touching in its majesty," an intriguing phrase that suggests both intimacy and grandeur. "Touching" scenes are often small and intimate, like a kid giving flowers to his sick grandmother. "Majestic" scenes are often large and public, like a snow-covered mountain or a king entering a throne room. The view from Westminster Bridge combines both this elements.
The speaker feels both awed by and close to the landscape.
He uses another colon: maybe now he'll stop keeping us in suspense and describe this amazing view.

This City now doth, like a garment, wear
The beauty of the morning;

We learn what time it is: London "wears" the morning like a nice coat or some other piece of clothing ("garment").
These lines hint that maybe the morning, not London itself, is responsible for the stunning quality of the view. As in, the garment could be so beautiful that it doesn't matter what the person wearing it looks like. Anyone could be wearing it, and you'd be like, "That's one heck of a garment, there."
Similarly, the word "now" shows that the beauty depends on the time of day. It's a fleeting, transient beauty. Maybe when the morning is over, and London is forced to change clothes, as it were, the speaker would think, "Oh. Now it's just London again. Been there, seen that." (There we go with our skepticism again.)

silent, bare,
Ships, towers, domes, theatres, and temples lie
Open unto the fields, and to the sky;

In general terms, the speaker describes some of the sights that are visible from Westminster Bridge.
The words "silent" and "bare" are positioned in the poem such that they could describe either the morning or the sights. Because of the semi-colon before them, the sights are the more obvious choice, but the ambiguity is important.
The setting is "silent" because of the early hour which, from Dorothy Wordsworth's journal, we know was around 5 or 6am.
"Bare" is an interesting word that means "naked" or "unadorned." It contrasts with the image of the city wearing clothing from line 4. Here, the ships and buildings are nude.
From Westminster Bridge in 1802, you could have seen a lot of the highlights of London, including the "ships" of the River Thames; the "dome" of the famous St. Paul's Cathedral, designed by the architect Christopher Wren; and the iconic Tower of London.
One thing you could not have seen in 1802, but that you could see today, is the Big Ben clock – it wasn't built yet.
Despite being all crowded together within one city, the speaker gives an impression of spaciousness by noting that the ships and buildings are "open" to the fields of London and to the sky.
One source points out that London had fields that were close to the city in 1802 but that no longer exist (source).

All bright and glittering in the smokeless air.

The speaker sums up the whole scene at the end of the poem's first chunk of eight lines, called an "octet."
He focuses on the early morning summer sunlight, which makes the buildings "bright and glittering." The word "glittering" in particular suggests that the scene is not static but rather constantly changing with the shifting light.
Our favorite word in the poem is "smokeless." What a word. He means that neither the characteristic London Fog nor smoke from chimneys obscures the bright light.
In London, as in San Francisco, it is common for fog to cover the city throughout the morning. The speaker is lucky to catch the city on a morning that is completely free of fog.