Be friendly and outgoing. Get to know as many people as you can, and make a good impression. People are more likely to invite you to parties if they like you and feel comfortable around you.
Get involved. Join clubs, sports, and other activities that you're interested in. This is a great way to meet new people and get invited to parties.
Be up for anything. Don't be afraid to try new things and go to new places. If you're always open to having a good time, people will be more likely to invite you to parties.
Be respectful of others. Always be polite and respectful of others, even if you don't know them. People are less likely to invite you to parties if you're rude or disrespectful.
Don't be too eager. When you're invited to a party, don't be too eager to accept. Wait a day or two to respond, so that you don't seem too desperate.
Dress appropriately. When you go to a party, dress appropriately for the occasion. This doesn't mean you have to wear a suit, but you should wear something that is clean, pressed, and appropriate for the setting.
Be a good guest. When you're at a party, be a good guest. Be polite, respectful, and have a good time. Don't get too drunk, and don't cause any damage.
Follow up after the party. After the party, send a thank-you note to the host or hostess. This is a nice way to show your appreciation for being invited.
As for whether you should just show up to a party, it's generally not a good idea. If you're not invited, it's best to respect the host's wishes and not go. However, there are some exceptions to this rule. If you're really close with the person throwing the party, or if it's a casual party where people are coming and going, it might be okay to just show up. But it's always best to err on the side of caution and get an invite before you go.