What are some alliterative lines in Beowulf?

Alliteration is the defining feature of poetry in Anglo-Saxon. Just as in the modern day our first thought about what makes a poem will probably be that it will rhyme—although this is not always the case—in the Anglo-Saxon period, poetry was defined by the use of alliteration to unite two half-lines, which are usually represented with a break or gap in between. Many translations of Anglo-Saxon literature attempt to preserve this alliteration, but translations have different purposes. Some seek to be especially true to the meaning, rather than the form, of the verse and will therefore not preserve the alliteration. For this reason, it is always preferable to read Beowulf and other Anglo-Saxon texts in the original language with a page translation if you need one. If you look at the poem in Anglo-Saxon, you will see this alliteration is continually present throughout the poem. Here is an example in the opening two lines:

Hwaet! We Gardena in geardagum
þéodcyninga þrym gefrúnon

In modern English, this does not alliterate. It is something like, "So! We, the Spear-Danes, in the olden days, / heard tell of the glory of the clan kings." But in Anglo-Saxon, the alliteration is an essential part of the poetic form.
Look at the poem in Anglo-Saxon in the link below, and you will see that this continues throughout the poem.
http://www.heorot.dk/beowulf-rede-text.html

Beowulf is written in alliterative verse. The poetic unit of the poem is a single line that is divided by a strong pause (known as a caesura) and that employs alliteration in some of the line's stressed words. As such, basically any line in Beowulf uses some kind of alliteration (the same sound used at the beginning of words). A prime example of alliteration can be seen in the fourth line of the poem: "There was Shield Sheafson, scourge of many tribes" (4). To make the alliteration easier to see, I'll paste the line below with the alliterative elements marked in bold:

The was Shield Sheafson, scourge of many tribes...

The "s" sound repeated throughout the line is a clear example of alliteration. The use of alliteration was important for the structure of Old English poetry, and so the poet of Beowulf was following a common trend when structuring the verse of the poem. Once you get the hang of it, alliteration is easy to identify, so I'd encourage you to check out the rest of the poem and see what kind of alliteration you can find.