Realistic Expectations- Why you can't gain 20kg of muscle before your holiday next month
Your body can only build a limited amount of muscle in a given time frame, you need to accept that biological reality in setting your goals. If you eat more calories than your body can use in the process of building muscle then it will end up as fat which I doubt is what you are going for.
If you read the weight gain section it will give you advise on how to gain 0.5kg/1lbs a week. More than that and you are and the amount of fat you gain really won't be worth any additional muscle gain you might get.
Training for Strength/Muscle Gain at Home
Is the problem that you find the thought of going into a gym intimidating?
If so, you basically need to suck it up and give it a go. Nobody will be judging you. We were all noobs once upon a time and the vast majority of lifters are glad to see more people getting into it. But mostly, nobody will find you very interesting so are very unlikely to notice what you are doing at all So you're probably building it up to be worse than it will be.
It can help to have a plan and know what to expect, for example, know you are going to go in, find a rack and use it to squat 40kg 5x5. You may well feel more self conscious if you haven't already decided what to do and end up standing around thinking- but even then nobody will actually care what you are doing.
And FWIW some of the nicest people I've ever met are huge, strong guys who can be quite intimidating to watch lift.
Or do you think you should train at home to get a 'base' first so you aren't the smallest and weakest?
You might well be the smallest and weakest but you won't be for long and there will be other beginners about. And that guy benching 100kg- it's probably not that long ago he was struggling with 50kg and because your goals forever increase, he probably doesn't think of himself as especially strong.
And anyway, this is flawed logic, it's harder to make good gains at home because of the lack of equipment so you could just be setting yourself up to fail. Plus postponing going to the gym probably isn't going to make it any easier the first time, it goes back to you needing to just suck it up and give it a go because it's not bad at all.
Or do you think you can train with just body weight?
You can do a fairly decent upper body routine with just body weight exercise though the main problem is training legs. If your legs are weak then you will never be strong as a whole. If your legs are twig shaped and your upper body gets decent then you'll look silly in shorts. Plus, even for upper body it is much simpler to use weights and you can still have bodyweight exercise within your lifting routine.
Or do you plan to buy equipment for a home gym?
This is a valid option so long as you are really willing/able to make the required investment. To make proper gains you should be doing one of the routines recommended here. For them you will at least need:
Squat rack, preferably with safety bars, that goes low enough to bench from (£130)
20kg bar (£90)
100-120kg in weights (£200)
You may also want:
Rubber floor matting- to protect it from weights being dropped on it
Pull up bar
And you'll be surprised how quickly you need more weight. In the brackets is a rough idea of what it would cost new but you can save money if you get lucky on gumtree/ebay. The new costs are for the cheap end of the market, personally if I were to be setting up a home gym I'd spend a bit more to have it work and last well.
You'll notice I didn't say go to Argos and get a 50kg weights set. You can't bench or squat properly with no other equipment and 50kg is nowhere near enough to be challenging for long enough to make any noticeable progress.