I recently got a friends magic collection as he didn't use it anymore and didn't have space to store it. It's only quite old cards but a lot of them. Is there any point in keeping this collection. As I said I'm completely new to mtg. If there is any use of it, how do I start assembling a deck?
How old are the cards? The really old sets are actually kind of bad (relative to now) with regards to playable cards if you don't have the powerful cards that everyone cares about from those sets (Duals, Power, Legendary Creatures, unique and powerful lands, etc.) but there is also a lot of chaff in the older sets that don't really see any play today. Either because they have been outpaced by new cards or because they weren't all that good to begin with. There are some middling cards but not a lot. Can you provide pictures or a list of cards (even a small sample list)?
So, with older cards especially, Vintage, Legacy, and EDH/Commander are likely the ones you can at least look towards. Legacy and Vintage are higher in power level so it would be tougher to break into without some of the more expensive cards (you can do it without them of course) and EDH would require a playgroup to play with.
Modern might be viable depending on how old we are actually talking. Pauper (if you can find a place that plays it) could be a reasonable format to get into. Or, if you have a few friends who want to play, you can use the cards to try to all build similarly powered decks with your collection to just play among each other.
If you do have a group of friends, especially ones who are also just starting, just sitting down and building different decks from the cards could be the way to start. They may not be as tuned as they could be, but it allows each person to get practice with deckbuilding, practice with understanding the rules (though the older cards may have updated text online that could make things hard to follow), and an exposure to different strategies so you can determine if there is a particular strategy you happen to enjoy over others.
The Gatherer website used to let users enter a rating of 0 to 5 stars for each card. The newer cards don't have ratings but the older cards still have their ratings. Even though the ratings are the opinions of MTG players and aren't factual, it could help to give you a feel for which cards are good. Just go to gatherer.wizards.com and enter a card name in the search field then search. You should see a community rating under the card picture.
Once you start getting into Magic, you'll see that there is no such thing as having too many cards.