30 minutes of walking per day is a great place to start, but do have a look at the Couch to 5K plan (link) as it will give you more structure for progression.
I'd recommend a food and activity diary. Write down everything (everything) you eat and drink, how much, where you were/who you were with, and how you felt (hungry, bored, tired, etc.) - it's always interesting to spot patterns in how you eat depending on the context of what's going on around you, and it helps to highlight any issues around emotional/habitual eating. Add in details on what activity you're doing, and you'll have a record of your progress which can be quite motivating in moments of 'can't be bothered'