I did construction a little, it's good pay, but you have to be able to work real hard, it can be dangerous sometimes, but if you have your wits about you (as well as others keeping safety measures), you'll be fine. Muscle helps a great deal, but sometimes you can do without it. Restaurant work is physically easy, but it can get very stressful at times, you'll need to be patient and work well under pressure (which is sometimes the case in construction as well). Gardening (I can't speak from experience cause I've never worked as a gardener) seems like it would be easy to do as long as you got the creativity and what not, if this is what you enjoy doing you should go do it, even if only as a side job.
Bottom line (if you are looking for a career, not just a first job), just do what you love doing, and you will be happy, if it doesn't make enough, you can always get a part-time job to supplement it. but ALWAYS do something you can enjoy doing, if you don't enjoy it, you'll always be unhappy and you won't do very well, and will be less likely to get raises and your work will be shoddy. “Choose a job you love, and you will never have to work a day in your life.” ? Confucius
Depends on the type of construction. If you're just a helper, you're only going to make $10-12 an hour. As you gain a skill set and move up, you can make considerably more. Restaurant work varies as well - if you're a busser, host, cook, or dishwasher then don't expect to make as much as a server/waiter/waitress unless that restaurant splits tips evenly. Gardening - do you mean landscaping? If so, same as construction. If not, probably not as lucrative.
depends on what you do and geography. in sunny florida, you could make a lot of money gardening and landscaping and you could do this pretty much all year. restaurant is your only chance at working all year 12 months straight because its indoors. construction normally don't work in the winter. but i think the pay is pretty good.