You can search your collection for a particular card or cards using the search bar, located in the top left of your Collection page.
Type what you're looking for into the search bar and press Enter. By default, the search bar searches for card names that match the search. For example, searching "Jace" would display all your copies of Jace planeswalkers (and any other card with Jace in the name, such as Jace's Phantasm).
You can change what the search bar searches for using text commands. The following options are available:
search by set - to display cards from a particular set, type "s:" (without quotes) into the search bar. A list of sets will appear below and typing more will display sets that match. For example typing "s:commander" would display all the Commander sets. Select a set from the list and cards from that set will be displayed.
search by foil - to display cards that are foil or non-foil, type "f:" (without quotes) into the search bar. True or false will appear as options below. Selecting true will display all foil cards in your collection, selecting false will display all non-foils.
search by colour - to display cards of a particular colour, type "c:" (without quotes) into the search bar. A list of colours will appear below. Selecting one will display all cards that have that colour.
You can also combine these text commands. For example, you could display all your blue foil cards from Return to Ravnica by searching "s:Return to Ravnica f:true c:blue".
We plan to extend the text commands to cover any search criteria you may need. Let us know if we're missing anything you're after and we'll add it in as soon as possible.