Don't know how to start a conversation.

Maybe try to think of some things first such as “what are you having for dinner tonight?” Or “what are you doing at the weekend?” Etc and then you might discover something you can expand on and chat about 

Just say whatever your heart desires. Be kind that's all. Keep it simple. Don't overthink it.

I just use my powers of observation and sometimes point out something non-obvious that other people have missed.

It creates a talking point and gives them something to think about.

You need to do the "hello" bit first though.

JAX43 said:

i want to cheer up so would rather talk about funny things

Think back to when you were younger- what things used to bring you the most joy?

What things still make you grin now when you see them? I find puppies are an inexhaustable supply of sheer joy to see and play with.

I’m quite good at meeting new people and starting a conversation - getting to know someone sometimes anyway if I can identify things to ask about or if they like similar things to me….. I find it difficult making conversation with people I’ve known for some time - I prefer to have a comfortable silence. I’m also not good at stating conversations with people I semi know at the kettle and would rather run away 

I had a really unusual doll whose eyes changed colour ( including orange and purple)

That's hard. It can be difficult to find joy in things when in that particular hole, I know, if you can remember something that once was interesting to you it can be useful? Even if you have to go right back to childhood and talk about a toy you once had?

I had dinosaurs as a kid I really liked to play with. I still remember all the details about them, name, colour, textures.

I feel really lonely and sad but i want to cheer up so would rather talk about funny things but can't think of anything because i'm too unhappy.

Geeky Gothic Guy said:

When we have new starters at work, besides the “y’alright” comment, that’s about it. I haven’t a clue.

How about:

"what made you want to work here?"

"What did you do before coming here?"

"How are you finding it here?"

"It sometimes takes a fresh pair of eyes to spot the areas for improvement. What would you change here if you had the chance? "

I’m the same. When we have new starters at work, besides the “y’alright” comment, that’s about it. I haven’t a clue. I used to be able to start conversations a long time ago, but since I moved up here to Lincolnshire 10 years ago, my social skills have declined massively, and I find it hard to make friends. Most of the people I do talk to at work, started with a conversation about mental health. No, real advice I’m afraid, but you’re not alone.  

How about telling us the sort of situations you find yourself in (is it at work, a date, with family, at church etc) and we can give some pointers.

Typically something shared is a good starting point. For example

If you are sitting on a park bench with someone else and you see a dog running after a ball time after time - use that. Start with something like "look at that dog, it must be nice to have something so simple to make you happy. What would make you as happy as that?"

If you see a couple of people having an arguement then consider "I wonder what they are talking about - they look quite agitated. Maybe they need a puppy in their life - what do you think would make them calm down?"

It there is nothing much to see but the weather is looking a bit iffy you could try "if you could be in any climate right now, where would you want to be"?

The trick is to create a connection then leave it with an open ended question to draw them into the conversation.

It takes masking to start as we are likely to be out of our comfort zone initiating the conversation but once we have lit that touch paper then it should take on a life of its own.

When you get someone who has conversation skills as bad as yours then it can be really hard work and in these cases it helps to enjoy the silence - you can drop in a statement like "it is nice to be with someone where I don't need to be trying to make a conversation" to let them know you are ok with it being quiet and this can put them at peace.

Can you give us a bit more to work with and we can throw a few more meaningful suggestions your way?