Bodybuilding affects flexibility

Does weightlifting affect speed and flexibility?

Will I gain weight from lifting weights?

Yes and no. Lifting weights provides the stimulus your body needs to grow (gain muscle mass), but lifting on its own is of no use. Imagine that your caloric intake while lifting was 1000 kcal per day (extreme value, just to get the point across). If your caloric intake weren't enough to provide you with basic functions (exercise, temperature measurement, etc.), how should you gain mass?

How many calories should I eat then?

While I think higher levels were encouraged in the past, the state of the art on training days seems to be a calorie excess of around 500 kcal (depending on the source in the range of 300 to 600 kcal).

Correct macros

Another important factor is the macros. If you eat mostly fats and (especially) carbohydrates / sugars, you will find it difficult to gain muscle mass (you will most likely gain fat if you have an excess). You need to get some protein intake. There is a lot of ado about the exact protein / fat / carbohydrate ratios and I'm not going to make a recommendation here, but it won't be too hard to find baseball numbers online. Definitely avoid unprocessed foods as much as possible. Don't be overly dogmatic, but processed foods are (often) filled with poor quality nutrients that can adversely affect your progress.

Strength ≠ mass

I've talked about how to gain mass. Now let's see how we can avoid it. The first thing to note is that strength and muscles are not the same. You can get a lot stronger without massaging.

The effect of the repetitions

In addition to what you eat, another important factor is the number of repetitions per set. To gain strength without gaining mass, stick to lower repetitions. The strength program is about 4 to 6 repetitions (depending on the exercise), while the classic mass program includes about 8 to 12 repetitions.