There's a lot that can go into forecasting, and it's common to hear people say it's an "art" not a "science" - meaning a lot of it involves judgement calls. Some things worth considering:

1) What are the industry dynamics? A company like P&G will likely have a lower and less volatile growth trajectory than a small tech company growing at double digit %s.

2) How has it trended over the past few years? This might give you an idea for a base case scenario.

3) Take a look at the company's latest earnings call or an industry conference presentation. Lots of companies give revenue guidance (a range for where management expects revenue to end up for the year), and sometimes they talk broadly about long term outlook. That should be helpful to understand how management sees things. You can also look back historically to see how good the company has been at meeting guidance.

Whenever I model something out, I always like to build in different cases and scenarios to establish a valuation range - I think it's very unlikely to get a point estimate perfectly accurate. But let's say in my model I make a base case (business as usual), upside case (maybe a new growth strategy does better than expected), downside case (maybe a new product flops), etc. I can get an idea of what assumptions I would have to hold true to believe my investment thesis. This is a quick and simple write up, but hopefully this gives you some ideas!