The way that Sent folders are organised varies between email clients (such as Outlook Express, Thunderbird, Windows Live Mail, etc) and the multitude of web-based services available (such as Outlook.com, Gmail, Yahoo, GMX, etc). Since you've not told us what you're using it's hard to provide a definitive answer. However the following might help.
In many cases clicking on the 'To' column header (in your Sent folder) will put your emails into order by name. That should work with any email client (such as Outlook Express) but with some web-based services you have to use a drop-down 'Sort by' menu to get your sent emails into name order.
Once you've done that you'll have all of the relevant emails grouped together in a block. You now need to select the whole of that block. With most email clients (Outlook Express, etc) you can do that by clicking on the first email in the block and holding the Shift key down while clicking on the last one in the block. With web-based services (where you need to place a tick next to each email, rather than highlight it) you might be able to do something similar by placing the first tick and holding the Shift key down while placing the last tick; however you might need to tick each email individually.
Now that you've got the whole block of emails selected you simply need to use the 'Delete' function. With email clients (such as Outlook Express) you normally do that by right-clicking on any email in the block and selecting 'Delete'. With web-based services there will be a Delete button anyway.