Unlike with the Mac, in iOS there is no way to select text and search for it on Google in a new tab. There is, however, a workaround that brings about the same result on iOS.

Select text in Safari to bring up the context menu on iOS and then tap on Define. This brings up the dictionary panel for the selected text. If you’ve selected a word that is not in the dictionary, or multiple words or usually even if the word is in the dictionary, on the bottom right there is Search Web. Tapping that opens a new tab and searches for the selected text. This is equivalent to selecting text, copying, opening new tab, pasting the text in omnibar.

Things to note:
Even though the hint mentions Google, this uses the default search engine set in the Safari preferences so your preference is respected.
This is not specific to Safari. Any app that allows selection of text, and brings up the contextual menu will work with this by opening Safari. …

Source: Mac OSX Hints