Just take it slow and steady Petal. You can build your fitness up, but you need to be persistent, doing little but often.
3 years ago I was overweight and very unfit, but wanted that to change. I asked for a Fitbit for my birthday and that gave me some motivation. I started off small, just going out for short walks for maybe 15 minutes, but I tried to make sure I did that every day. Gradually I was able to increase the amount of time I was walking for and then I started to get faster. Within 3 months I was walking 2 miles in about 45 minutes and I'd lost 21 pounds. Now I regularly walk 3 miles and I often do 5-6 miles if I have time. I now do a mile in about 18-19 minutes. I've also just started to try running (well jogging really), which I never thought I'd be able to do again (I did a bit of running about 10 years ago because I committed to a 5k charity run but after that was over I stopped).
You can do this, just don't set unrealistic goals or you'll lose motivation if you can't achieve them