New to the forum advice on feeding birds

Hi I live in south London and have a small garden, I have very little knowledge about birds but I'm interested in trying to encourage them into the garden. In the last week or so I have seen what looks like a little blue tit in the garden. To be honest it's probably not a blue tit but it's a small bird with a bit of colour on it, possibly just smaller than a robin. Anyway I would like to encourage it into the garden, could someone advise me on the type of bird feeder and food for smalll birds. Thank you Sandra