1) I typically recommend teens lose weight at a slower rate than adults, just because you're still growing and developing, and that's not something you really want to mess with. However, I would conventionally have an adult lose 1-2lb/week (depending on what their starting point is - the bigger they are, the bigger the increments, the smaller they are the smaller the increments), so in 20 weeks they'd be looking at 20-40lb. You could probably do the lower extremity there if you're smart about what you do - although I'll inform you now that if you work on fat-loss correctly, you'll probably be quite happy with the results of even 10-15lb coming off. I'll get into the right things to do over the next few points.
2) Eat meat. For some reason everyone seems to think that meat makes them fat, and I suppose if you ate a whole turkey every day that died from drowning in gravy, yeah, you might gain some weight. But actual meat is relatively low in calories (unlike the fat around it, which you should still have some of). For pure meat, it's about 1kcal/g, maybe a little more. The fat around it is 9kcal/g, so be conservative with the fat, but don't eliminate it completely. I recommend having 200-300g meat daily. That includes meat of all colours and from creatures of the land, the sky and the sea. You could have chicken, ham or fish in your lunch and a steak in your dinner. Easy done. You can even have bacon and eggs for breakfast daily while losing weight, but again pay attenetion to how much fat you keep on the bacon.
Get some additional protein outside of meat from eggs and milk. 3 serves of a milk a day, minimum, preferably at times when you've just been in contact with direct sunlight or are about to say hi to the sun. Skim and trim milk may be preferable over full cream for the reduced fat content, but if you do drink full cream you can always reduce calories from other areas of your diet.
Eat lots of vegetables. When I have control over what's int he kitchen (not often the case), I like to have a salad at some point during the day, and some steamed vegetables in another meal. Relatively easy way to get your 5 serves a day. Be minimalistic on the calorie-dense vegetables, like potato. Corn's technically a grain, but it's also calorie-dense, so I wouldn't eat too much of it.
Eat some raw fruit each day. 2-3 serves is good. From personal experience, I'd follow up fruit with some nuts. I can have 3 serves of fruit in one sitting and be starving again 30min later. If I follow up my fruit with even a tiny dose of something like almonds, I'll be good to go for hours afterwards.
For both fruit and vegetables, the more colours you consume, and the deeper the colours, the better.
Paleo dieters treat all grains as hell-spawn. Advocates of the conventional food pyramid think grains are the bee's knees. I think both parties are wrong. Grains (including rice, wheat, sugarcane and corn, amongst others) don't typically have a lot of nutritional value, and they are mostly just a source of carbohydrates, so they're fairly empty calories. This means they're quite expendable (unlike how the food pyramid presents them), and they're the one area of the diet that I'd have people looking to lose weight cut back on out of their solid foods (although I'd have them cut back on soda first and foremost). But you can lose plenty of weight while having grains in your diet. I've done it successfully, as have many human beings for many years. If you currently eat a lot of grains (or stuff made from grains), I'd start with switching from highly processed/refined grains to whole grains first, and then think about cutting back on grains altogether if necessary.
Finally, make an allowance for some sanity food. It's okay to have pizza, lollies, ice cream, cake, cookies etc every now and again. Just do so in moderation.
3) I don't know what SlimFast is and am currently too lazy to ask google, so I can't make an informed comment. If it's just one of those meal replacement shakes, then I guess it depends on whether you like it or not. If you don't enjoy your food, you won't stick with it for very long. If it's a pill or supplement or whatever that's supposed to magically do the work for you, I wouldn't touch it with a 20-foot rusty halberd. In any case, it's definitely not essential.
4) Strength training is your friend, if the resources to do it are available. Everyone focuses on cardio for weight loss, but cardio doesn't preserve much muscle mass, and when you're losing weight, your body will literally get rid of whatever it deems least important for survival. Which would be better? To lose 20lb and have it be 10lb muscle and 10lb fat? Or to just lose 10lb fat? The latter, honestly. If you do the former, then you just become "skinny-fat," whereas if you do the latter you become this "toned" thing everyone's into these days (obviously, that depends on just how much fat you've got to lose; but you'll be closer to "toned" than "skinny-fat").
I don't know what resources/equipment you've got available, but in the best case scenario you'd be doing an even amount of squats (any variation that uses a controlled full range of motion), pushes (bench press, overhead press, dips, push ups, etc), pulls (pull ups, pull downs, rows) and hinges (deadlifts and variations thereof, hyperextensions, etc). If all you've got to work with is your bodyweight then you might just do bodyweight squats, box jumps, push up variations, inverted rows and glute bridges. The important thing is that you practice good technique, warm up properly, and seek progressive overload whenever possible (ie if you can do 10 incline push ups, work up to getting 10 from the floor; if you can do 10 squats, work up to doing 20, etc).
